Revert "Merge cpu_features build targets into //system_wrappers."

This reverts commit 2b242d8fba59ccf67e4c5bcf5a1ea80917a23e1c.

Reason for revert: Breaks downstream project.

Original change's description:
> Merge cpu_features build targets into //system_wrappers.
> 
> Before this CL, functions declared in cpu_features_wrapper.h where
> not defined in the same build target, causing brittle builds that
> might fail at link time if the binary was not depending on
> //system_wrappers (the target with the definitions), violating [1].
> 
> This CL moves everything into //system_wrappers and also moves
> cpu_features_wrapper.h definitions from C to C++ (in order to be able
> to add the definitions to a C++ build target like //system_wrappers).
> 
> [1] - https://webrtc.googlesource.com/src/+/refs/heads/master/style-guide.md#h-cc-pairs
> 
> Bug: None
> Change-Id: I5a0009cddb17206b19f2a71eeba722faacc4bcae
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/183380
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#32039}

TBR=mbonadei@webrtc.org,kwiberg@webrtc.org

Change-Id: I4daa7582e55a0343eef72f08ed023c73e0b6456b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: None
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/183443
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32040}

system_wrappers/BUILD.gn

@@ -16,7 +16,6 @@ rtc_library("system_wrappers") {
   visibility = [ "*" ]
   sources = [
     "include/clock.h",
-    "include/cpu_features_wrapper.h",
     "include/cpu_info.h",
     "include/ntp_time.h",
     "include/rtp_to_ntp_estimator.h",
@@ -31,6 +30,7 @@ rtc_library("system_wrappers") {
   defines = []
   libs = []
   deps = [
+    ":cpu_features_api",
     "../api:array_view",
     "../api/units:timestamp",
     "../modules:module_api_public",
@@ -50,8 +50,7 @@ rtc_library("system_wrappers") {
         "/nsprpub/pr/include",
       ]
     } else {
-      sources += [ "source/cpu_features_android.cc" ]
-      deps += [ "//third_party/android_sdk:cpu_features" ]
+      deps += [ ":cpu_features_android" ]
     }
 
     libs += [ "log" ]
@@ -59,7 +58,7 @@ rtc_library("system_wrappers") {
 
   if (is_linux) {
     if (!build_with_chromium) {
-      sources += [ "source/cpu_features_linux.cc" ]
+      deps += [ ":cpu_features_linux" ]
     }
 
     libs += [ "rt" ]
@@ -79,6 +78,10 @@ rtc_library("system_wrappers") {
   ]
 }
 
+rtc_source_set("cpu_features_api") {
+  sources = [ "include/cpu_features_wrapper.h" ]
+}
+
 rtc_library("field_trial") {
   visibility = [ "*" ]
   public = [ "include/field_trial.h" ]
@@ -108,6 +111,24 @@ rtc_library("metrics") {
   ]
 }
 
+if (is_android && !build_with_mozilla) {
+  rtc_library("cpu_features_android") {
+    sources = [ "source/cpu_features_android.c" ]
+
+    deps = [ "//third_party/android_sdk:cpu_features" ]
+  }
+}
+
+if (is_linux) {
+  rtc_library("cpu_features_linux") {
+    sources = [ "source/cpu_features_linux.c" ]
+    deps = [
+      ":cpu_features_api",
+      "../rtc_base/system:arch",
+    ]
+  }
+}
+
 if (rtc_include_tests) {
   rtc_test("system_wrappers_unittests") {
     testonly = true

system_wrappers/include/cpu_features_wrapper.h

@@ -13,6 +13,10 @@
 
 #include <stdint.h>
 
+#if defined(__cplusplus) || defined(c_plusplus)
+extern "C" {
+#endif
+
 // List of features in x86.
 typedef enum { kSSE2, kSSE3, kAVX2 } CPUFeature;
 
@@ -37,4 +41,8 @@ extern WebRtc_CPUInfo WebRtc_GetCPUInfoNoASM;
 // values in the above enum definition as a bitmask.
 extern uint64_t WebRtc_GetCPUFeaturesARM(void);
 
+#if defined(__cplusplus) || defined(c_plusplus)
+}  // extern "C"
+#endif
+
 #endif  // SYSTEM_WRAPPERS_INCLUDE_CPU_FEATURES_WRAPPER_H_

system_wrappers/source/cpu_features_linux.c

@@ -8,9 +8,9 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#include <features.h>
 #include <stdlib.h>
 #include <string.h>
+#include <features.h>
 
 #ifdef __GLIBC_PREREQ
 #define WEBRTC_GLIBC_PREREQ(a, b) __GLIBC_PREREQ(a, b)
@@ -21,10 +21,10 @@
 #if WEBRTC_GLIBC_PREREQ(2, 16)
 #include <sys/auxv.h>
 #else
-#include <errno.h>
 #include <fcntl.h>
-#include <link.h>
 #include <unistd.h>
+#include <errno.h>
+#include <link.h>
 #endif
 
 #include "rtc_base/system/arch.h"
@@ -36,7 +36,7 @@
 uint64_t WebRtc_GetCPUFeaturesARM(void) {
   uint64_t result = 0;
   int architecture = 0;
-  uint64_t hwcap = 0;
+  unsigned long hwcap = 0;
   const char* platform = NULL;
 #if WEBRTC_GLIBC_PREREQ(2, 16)
   hwcap = getauxval(AT_HWCAP);