This patch adds alignment information for long double to the 64-bit PowerPC

ELF subtarget.

The existing description string is moved from PPC64TargetInfo to its
DarwinTargetInfo subclass, to avoid any changes to the Darwin ABI.
PPC64TargetInfo now has two possible description strings: one for FreeBSD,
which requires 8-byte alignment, and a default string that requires
16-byte alignment.

I've added a test for PPC64 Linux to verify the 16-byte alignment.  If
somebody wants to add a separate test for FreeBSD, that would be great.

Note that there is a companion patch to update the alignment information
in LLVM, which I am committing now as well.

llvm-svn: 166927