Provide a generic fallback for pg_compiler_barrier using an extern function.

Provide a generic fallback for pg_compiler_barrier using an extern function.

If the compiler/arch combination does not provide compiler barriers,
provide a fallback. That fallback simply consists out of a function
call into a externally defined function.  That should guarantee
compiler barrierer semantics except for compilers that do inter
translation unit/global optimization - those better provide an actual
compiler barrier.

Hopefully this fixes Tom's report of linker failures due to
pg_compiler_barrier_impl not being provided.

I'm not backpatching this commit as it builds on the new atomics
infrastructure. If we decide an equivalent fix needs to be
backpatched, I'll do so in a separate commit.

Discussion: 27746.1420930690@sss.pgh.pa.us

Per report from Tom Lane.