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.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@214012 91177308-0d34-0410-b5e6-96231b3b80d8
builtins: re-enable building assembly sources

Re-apply SVN r213684 which was reverted in SVN r213724 since it broke the
build bots.  Add a tweak to enable inclusion of the assembly sources in
standalone build as well.

Original commit message:

This patch address the PR20360. The CMake assembler build system
ignores the .S assembly files in builtins library build. This patch
fixes the issue.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@214013 91177308-0d34-0410-b5e6-96231b3b80d8