[builtins] Define fmax and scalbn inline

Define inline versions of __compiler_rt_fmax* and __compiler_rt_scalbn*
rather than depend on the versions in libm. As with
__compiler_rt_logbn*, these functions are only defined for single,
double, and quad precision (binary128).

Fixes PR32279 for targets using only these FP formats (e.g. Android
on arm/arm64/x86/x86_64).

For single and double precision, on AArch64, use __builtin_fmax[f]
instead of the new inline function, because the builtin expands to the
AArch64 fmaxnm instruction.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D91841