__builtin_sinf and friends problem

[David Edelsohn]

	Is __builtin_sinf() always suppose to hide the actual source of
those functions?  Or is it only suppose to be used when configure finds
they exist?  I think the latter, but a V3 header is using various builtin
functions without checking.

	c_std/bits/std_cmath.h uses __builtin_sinf, __builtin_cosf,
etc. without any #ifdef protection.  c_shadow/bits/std_cmath.h only uses
those functions if they exist and provides non-builtin and fallback
static_cast<double> versions.  Systems without sinf() linkage are seeing
undefined symbol linker errors when using float methods from <cmath>, such
as testsuite/26_numerics/c_math.cc

David