[cxx-mem-model] Rename __sync_mem to __atomic

[Andrew MacLeod]

In order to align with a standard naming convention, this patch renames 
all the __sync_mem references to __atomic.

The only real addition is in builtins.c::is_builtin_name needed to know 
that __atomic_ is also a builtin prefix.

I didn't include the testsuite changes since they are HUGE, and very 
rote...  they also included renaming all the sync-mem-* test file names 
to atomic-*

bootstrapped on x86_64-unknown-linux-gnu and no new regressions.

Andrew
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