TLS support for (almost) all supported archs

TLS support for (almost) all supported archs

[Rich Felker]

As of the latest commit, thread-local storage is working for i386,
x86_64, ARM, MIPS, and is theoretically partially working for
microblaze (I have no test platform for it yet, and some of the
dynamic linker relocation stuff is missing for some usage cases).

This brings us a lot closer to the next release, in which TLS and
microblaze support (hopefully for both of them together!) will be the
highlights. Most testing so far has been dumb test-cases, so if
anybody has real-world apps that make heavy use of TLS that would be
good trials to see if TLS is "really working", please try and let me
know how it goes.

One further change I've been considering and might try to get in
before this release is C++ ABI compatibility. It looks like not many
types would need changing to make musl name-mangling-ABI-compatible
with C++ apps linked to glibc, but I haven't looked into the issues in
detail yet. If you're looking for something to help out with, figuring
out how extensive the changes would need to be would be a very helpful
task (and would help determine if it's feasible at all, as well as if
it's feasible to target in the next release).

Rich