This adds support for building for mips64* architectures by using the
--use-cc-builtins configure option for ConcurrencyKit. Which is a
temporary workaround until proper MIPS supported is implemented in CK
upstream.
Closes GH-318.
Use ax_gcc_archflag.m4 to try to guess the most optimal -march value for
the target architecture, unless cross-compiling, or the value is set
explicitly with --with-gcc-arch.
Also, AArch64/ThunderX-specific updates to ax_gcc_archflag.m4.
In sb_concurrency_kit.m4, add --enable-lse to CK build flags if LSE
instructions are supported by the target.
