builtins: make the x86_64 compatible with GAS

The .rodata directive was added on the IA-64 (Itanium) platform.  The LLVM IAS
supports the .rodata on i386 and x86_64 as well.  There is no reason to really
restrict compilation of the builtins to just clang.  By explicitly indicating
that the data is meant to be pushed into the .rodata section via the .section
.rodata, the assembly is made compatible with clang and gcc (with GAS).

This will enable building these routines on the Linux buildbots via CMake.

llvm-svn: 214012