Re-add ALLOW_MIPS_SHARED_TEXTREL, sprinkle it around

Diff partially stolen from CheriBSD; these bits need -Wl,-z,notext in order
to build in an LLVM world. They are needed for all flavors/sizes of MIPS.
This will eventually get fixed in LLVM, but it's unclear when.

Reported by: arichardson, emaste
Differential Revision: https://reviews.freebsd.org/D21696