FreeBSD/mips64 uses the same shadow offset as MIPS64 on Linux.
The LLVM changes to use the correct shadow offset on FreeBSD/mips64 are in D49939.
Rather than adding '&& SANITIZER_LINUX' to the #ifdef for each architecture as they are ported to other systems, we might consider wrapping all of the 'internal_clone' implementations in a single '#if SANITIZER_LINUX'.
__syscall is valid for all FreeBSD platforms.
NetBSD will switch away from the syscall(2)/__syscall(2) approach and use libc natively. These indirect syscall functions are not portable enough and generate problems for various ABIs.
Sorry for the delay. This does still need review from folks familiar with sanitizers. I haven't yet rebased it to a more recent tree due to lack of review feedback. If there is interest in reviewing it I can update it to a newer tree and retest (or if there is feedback on the current patches I'm happy to update them).