libstdc++: Silence irrelevant warnings in <experimental/simd>

libstdc++: Silence irrelevant warnings in <experimental/simd>

Avoid
-Wnarrowing in C code;
-Wtautological-compare in unconditional static_assert (necessary for
faking a dependency on a template parameter)

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd.h: Ignore -Wnarrowing for
	arm_neon.h.
	(__int_for_sizeof): Replace tautological compare with checking
	for invalid template parameter value.
	* include/experimental/bits/simd_builtin.h (__extract_part):
	Remove tautological compare by combining two static_assert.