Build fix for archs where 64-bit __sync operators are not implemented.

[julianaito]

On my OpenBSD/macppc box, wrk fails to build at linking time:

stats.c:(.text+0xb20): undefined reference to __sync_fetch_and_add_8
stats.c:(.text+0xb30): undefined reference to __sync_fetch_and_add_8
stats.c:(.text+0xb70): undefined reference to __sync_val_compare_and_swap_8
stats.c:(.text+0xbe4): undefined reference to __sync_val_compare_and_swap_8

On some platforms, like ppc32, gcc does not implement legacy 64 __sync_* functions, it instead supports __atomic_* ones, introduced from gcc-4.8.

I've simply fixed the build by applying the same technique used in atomicvar.h where there is already a dual support of __sync_* and __atomic_*, and used an existing Debian patch, that i've synced against master.

(cc @fcambus)