Add --unwindlib=[libgcc|compiler-rt] to parallel --rtlib=

Summary:
"clang++ hello.cc --rtlib=compiler-rt"

now works without specifying additional unwind or exception
handling libraries.

Reviewers: rsmith

Subscribers: srhines, dberris, aheejin, llvm-commits

Differential Revision: https://reviews.llvm.org/D57128

llvm-svn: 352404