libstdc++: Don't fail if math_errhandling is not defined

Older glibc does not define math_errhandling with -ffast-math, in which
case floating-point exceptions are not used.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd.h (__floating_point_flags): Do
	not rely on the presence of the math_errhandling macro.