Fix MIPS TLS support

[kan]

Summary

Account for TLS_TP_OFFSET in thread pointer and for TLS_DTP_OFFSET
in dvp entries. This is identical to how powerpc lays out its
TLS, so shamelessly copy their code where appropriate.

Test Plan

qemu-mips-static env is able to build pkg now

[kan]

The alternative approach of fixing sysarch(GET/SET_TLS) in qemu-bsd-user might be called for, pending confirmation from sbruno.
Our kernel does unnatural things in rdhwr emulation codem qemu-user might have to follow.

I am not sure our kernel is correct though, iff there is ever an straightforward hardware implementation of rdhwr, it will unlikely to do the same magic our kernel does.

[sbruno]

This fixes my ability to use unmodified qemu-user code to build mips/mips64 packages.

[emaste]

Fine with me, make sure to include Relnotes: YES and comment on ABI concerns in the commit message.

[kan]

Updates diff to remove hdwr value fixup

[kan]

Abandon in favor of fix in qemu. Changing ABI is too heavy-handed.