[compiler-rt] Support for passing through linker flags to libc++ build
[compiler-rt] Support for passing through linker flags to libc++ build

This may be needed when cross-compiling to certain platforms.

Differential Revision: https://reviews.llvm.org/D54027

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@346063 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt