mips: csu: fix compilation w/ LLVM

GCC issues the warning, but with LLVM it is fatal- no matching .cprestore
with .cpload. Reserve some place on the stack and and add the proper
.cprestore to pair it with.

nop added in the !o32 branch to fill out delay slot instruction, just in
case.

Reviewed by: arichardson
Differential Revision: https://reviews.freebsd.org/D21697