<cmath>: Use ceil/floor/round/etc. intrinsics

In VS 2019 16.8 Preview 3, the following intrinsics will be available (MS-internal MSVC-PR-265316):

double __ceil(double);
float __ceilf(float);
double __floor(double);
float __floorf(float);
double __round(double);
float __roundf(float);
double __trunc(double);
float __truncf(float);
double __copysign(double, double);
float __copysignf(float, float);
unsigned int __signbitvalue(double);
unsigned int __signbitvaluef(float);

These are available for all architectures, declared in <intrin0.h>. We should take advantage of them. (We may need to report this to the UCRT too.)

Also tracked by DevCom-987080 (MS-internal VSO-1101527 / AB#1101527) and DevCom-1038270 (MS-internal VSO-1129605 / AB#1129605).

Start with the implementation and the intrinsic declarations in <intrin0.h>, then trace how the listed rounding, sign, and sign-bit operations are currently handled. Check support across all architectures and determine whether the corresponding UCRT work also needs reporting. Done means the relevant operations use the available intrinsics and the associated behavior is validated.