[compiler-rt] [builtins] Detect _Float16 support at compile time

Instead of detecting `_Float16` support at CMake configuration time,
detect it at compile time by checking for the predefined (by the
compiler) macro `__FLT16_MAX__` instead.

This solves the issue where compiler-rt is built simultaneously for both
x86_64 and i386 targets, and the CMake configuration uses x86_64
compilation to detect `_Float16` support, while it may not be supported
by the i386 target (if it does not have SSE2).

While here, rename `COMPILERT_RT_HAS_FLOAT16` to `CRT_HAS_FLOAT16`, to
conform more to the naming style used in `int_lib.h` and `int_types.h`.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D130718