go: Fix up go.test/test/fixedbugs/bug207.go failure [PR109258]

The PR109086 r13-6690 inline_string_cmp change to
      if (diff != result)
        emit_move_insn (result, diff);
regressed
FAIL: go.test/test/fixedbugs/bug207.go,  -O2 -g  (internal compiler error: in emit_move_insn, at expr.cc:4224)
The problem is the Go FE doesn't mark __builtin_memcmp as pure as other FEs,
so we ended up with
  __builtin_memcmp (whatever, whateverelse, somesize);
in the IL before expansion and the expansion ICEd on it.
As the builtin calls a library function which is pure or is inline expanded
as such, not marking it pure is an unnecessary pessimization from the FE
side, keeping such dead calls in the IL if they aren't needed will not help
anything.

The following patch fixes that.  Initially I've added just DECL_PURE_P to
it, but that unfortunately broke bootstrap, for __builtin_memcmp there is
also __builtin_memcmp_eq registered by the middle-end code if not registered
earlier and that one is registered with the usual flags (pure, nothrow,
leaf), so if __builtin_memcmp from FE was just pure, it would appear in the
IL as that it can raise exceptions and when folded into __builtin_memcmp_eq
all of sudden it couldn't and we'd ICE in verification.

I think tons of functions should have builtin_nothrow as well, but changing
that wasn't necessary for this fix.

2023-03-24  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/109258
	* go-gcc.cc (Gcc_backend): Add new static data members builtin_pure
	and builtin_nothrow.
	(Gcc_backend::Gcc_backend): Pass builtin_pure | builtin_nothrow for
	BUILT_IN_MEMCMP.
	(Gcc_backend::define_builtin): Handle builtin_pure and builtin_nothrow
	in flags.