Add support for __gnu_mcount_nc as the pg interface

This adds support to optionally support using `__gnu_mcount_nc` as the mcount
interface rather than `mcount` for Linux and EABI.  The other targets do not
provide an implementation for `__gnu_mcount_nc`.  This can be activated via the
`-meabi gnu` flag.

Resolves PR23969.

llvm-svn: 265888