Set -mlong-calls where needed to build clang/lldb

[andrew]

Summary

This adds -mlong-calls to the ARM builds to the llvm libraries, libc++, clang,
and lldb. This is needed for llvm 3.8.0 as the size of a static clang and
lldb is too large for a branch instruction.

I expect this will also be needed for Thumb-2 builds, but I have yet to test
this case.

[dim]

LGTM, with this applied buildworld TARGET=arm completes successfully with the clang380-import branch.