This patch, on top of D120048 <https://reviews.llvm.org/D120048>, supports
GetTls on Solaris 11.3 and Illumos that lack `dlpi_tls_modid`. It's the
same method originally used in D91605 <https://reviews.llvm.org/D91605>,
but integrated into `GetStaticTlsBoundary`.
Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86_64-pc-linux-gnu`.
Differential Revision: https://reviews.llvm.org/D120059
Rainer Orth