Add support for musl-libc on Linux

[Joerg Sonnenberger via cfe-commits]

On Sat, Jun 11, 2016 at 03:14:38PM +0800, Lei Zhang via llvm-commits wrote:
> Hi,
> 
> The first patch introduces musl as a new environment type in LLVM; the
> second helps clang to find musl's dynamic linker, which has a
> different naming scheme from glibc's.
> 
> The two patches together enable clang to support targets like
> "x86_64-pc-linux-musl" and build binaries against musl-libc instead of
> glibc. This make it easy for clang to work on some musl-based systems
> like Alpine Linux and certain flavors of Gentoo.

LGTM, but needs test cases.

Joerg