[compiler-rt] [builtins] Support conversion between fp16 and fp128

This patch adds both extendhftf2 and trunctfhf2 to support
conversion between half-precision and quad-precision floating-point
values. They are enabled iff the compiler supports _Float16.

Some notes on ARM plaforms: while __fp16 is supported on all
architectures, _Float16 is supported only for 32-bit ARM, 64-bit ARM,
and SPIR (as indicated by clang/docs/LanguageExtensions.rst).  Also,
__fp16 is a storage format and promoted to 'float' for argument passing
and 64-bit ARM supports floating-point convert precision to half as
base armv8-a instruction.

It means that although extendhfsf2, truncdfhf2 __truncsfhf2 will be
built for 64-bit ARM, they will be never used in practice (compiler
won't emit libcall to them). This patch does not change the ABI for
32-bit ARM, it will continue to pass _Float16 as uint16.

Differential Revision: https://reviews.llvm.org/D91732