math/big: fix incorrect register allocation for mipsx/mips64x
According to the MIPS ABI, R26/R27 are reserved for OS kernel, and may be clobbered by it. They must not be used by user mode.
See Figure 3-18 of MIPS ELF ABI specification: https://refspecs.linuxfoundation.org/elf/mipsabi.pdf
Fixes #73472
Change-Id: Ifda692a803176bfaab2c70d6623636c5d135f42e
Reviewed-on: https://go-review.googlesource.com/c/go/+/667816 Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Keith Randall <khr@google.com>