Override the TLS model when building mips64 binaries and static libraries

GCC uses "dynamic" TLS models when -fpic or -fPIC is explicitly
specified on the command line (which is only true for shared libraries).
It uses "static" (or "exec") TLS models otherwise. In particular, GCC
does _not_ use dynamic TLS models when PIC is implicitly enabled (which
it is on MIPS), only if a PIC flag is explicitly provided.

llvm uses "dynamic" TLS models if PIC is enabled either via a PIC flag
or if it is implicily enabled (as on MIPS64). This means that llvm on
MIPS64 always uses "dynamic" TLS models. However, dynamic TLS models
do not work for static binaries and libraries as the __tls_get_addr
function they invoke is only defined in rtld.

Written by: jhb
Reviewed by: arichardson
Differential Revision: https://reviews.freebsd.org/D21699