Later gcc and clang have deprecated =v (which maps to a specific temp
register) and instead we should just use =r to have the assembler
(hopefully!) save/restore things appropriately after choosing
a register.
Tested:
* AR9344 SoC, with userreg support
* AR9331 SoC, with no userreg support
Sponsored by: Sponsored by: DARPA, AFRL (MIPS TLS user register work)