[compiler-rt] Don't require c++ headers when configuring compiler-rt builds

Summary:
A sysroot without c++ headers is able to build compiler-rt, don't
require them when configuring available architectures from cmake.

Reviewers: samsonov, beanz, compnerd

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D22469

llvm-svn: 276151