Reduce large list of macros to the primary platform macros. Distingiush
between ELF (Linux, FreeBSD, NetBSD) and OSX as platform for the
assembler dialect.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186252 91177308-0d34-0410-b5e6-96231b3b80d8