Set correct .type directive for asm functions.

This is required per binutil manual.
https://sourceware.org/binutils/docs/as/Type.html#Type

Otherwise, runtime SEGV_MAPERR if linked by GNU bfd linker.

BUG=b/20218337
R=wzhong@chromium.org

Review URL: https://webrtc-codereview.appspot.com/47059004

Patch from Wei Zhong <wzhong@chromium.org>.

Cr-Commit-Position: refs/heads/master@{#9106}

webrtc/system_wrappers/interface/asm_defines.h

@@ -38,6 +38,9 @@ bl _\name
 .hidden \name
 .endm
 .macro DEFINE_FUNCTION name
+#if defined(__linux__) && defined(__ELF__)
+.type \name,%function
+#endif
 \name:
 .endm
 .macro CALL_FUNCTION name