implement atomics with gcc intrinsics

Gcc after 4.7 provides atomic builtins[1]. Use these instead of adding
yet-another-assembly port for Aarch64 (64-bit ARM). This patch enables
succesfully building and running atomicops unittest on Aarch64.

This patch enables using gcc builtins only when no assembly
implementation is provided. But as a quick check, atomicops_unittest
and rest of testsuite passes with atomicops-internals-gcc also
ARMv7 and X86_64 if the ifdef in atomicops is adjusted to prefer
the generic implementation.

[1] http://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
chmod -x configure.ac

Because configure.ac is not really executable. And because it
interferes with tab completion of configure.
allow asking for gcc atomics on all platforms

I.e. by doing ./configure CPPFLAGS=-DTCMALLOC_PREFER_GCC_ATOMICS
removed irrelevant comment