[WebAssembly] Support f16 libcalls

Add support for f16 libcalls in WebAssembly. This entails adding signatures
for the remaining F16 libcalls, and renaming gnu_f2h_ieee/gnu_h2f_ieee to
truncsfhf2/extendhfsf2 for consistency between f32 and f64/f128 (compiler-rt
already supports this).

Differential Revision: https://reviews.llvm.org/D61287

Reviewer: dschuff
llvm-svn: 359600