Fix arm_neon.h and arm_fp16.h generation for compiling with std=c89

Summary:
The inline attribute is not valid for C standard 89. Replace the argument in the generation of header files with __inline, as well adding tests for both header files.

Reviewers: pbarrio, SjoerdMeijer, javed.absar, t.p.northover

Subscribers: t.p.northover, kristof.beyls, chrib, cfe-commits

Differential Revision: https://reviews.llvm.org/D51683

test/Headers/arm-fp16-header.c
test/Headers/arm-neon-header.c
utils/TableGen/NeonEmitter.cpp

llvm-svn: 341475