This was shamelessly stolen from @jhb's MIPS/LLVM branch after discovering that LLVM still does this wrong. Specify initial-exec for MIPS64, as the default is currently (incorrectly) dynamic TLS without -fPIC.
The original commit was unclear- this was addressing some build breakage, yeah? mips64 built fine without it, but it inadvertently rode in with the TLS fixes that I merged in locally after discovering that the mips64 world was completely hosed.
Yes, for some reason Clang and LLVM get a bit confused about PIC; -fno-PIC gives:
clang-9: warning: ignoring '-fno-PIC' option as it cannot be used with implicit usage of -mabicalls and the N64 ABI [-Woption-ignored]
and thus the compiler thinks it's doing full-blown PIC so has to assume global dynamic TLS (or local dynamic for non-preemptible symbols). This works on glibc-based systems because their static libc has a simple __tls_get_addr implementation that just does what the initial exec implementation would have done had it been inlined (but with the additional pointless module index in the GOT that will always be 1), but our __tls_get_addr is instead a stub that returns 0. So it's a combination of a Clang/LLVM stupidity and an unhelpful libc on our part.
(and yes, MIPS doesn't do TLS linker relaxation with any linker, otherwise this wouldn't be noticeable)
Yes, the integrated assembler has been the default everywhere on MIPS since LLVM r348934/r348935 (part of 8.0.0), but was also enabled by default on FreeBSD since LLVM r336004 (part of 7.0.0), so this is no longer needed unless you want to support older compilers.
(FWIW, to complete the picture, -fPIE *is* accepted since it's still a position-independent code model and thus doesn't upset -mabicalls (though really PIC and ABI calls shouldn't be conflated), and thus overrides the implicit default -fPIC, giving initial exec as the default TLS model, so that might be a nicer way to go...)
The original log message at https://github.com/freebsd/freebsd/commit/0e0da5913af0ce4250d4484b28ac2450760c6e68 might be better as it has more detail. You would drop the last line about fixing it upstream, but the rest of the commit log explains the reasoning for this change.