On the x86_64 self hosted backend, thread locals are accessed through
__tls_get_addr on PIC. Usually this goes through a fast path which does
not lose any registers, however in some cases (notably any dlopened
library on my machine) this can take a slow path which calls out to C
ABI functions

Catch this case and backup registers as necessary

Fix a few other ones while we're here. Credit to mlugg

Fixes #30183