csu: Avoid additional nops in the MIPS INIT_CALL_SEQ macro

Since we had a .set reorder, the nop after the "jal" was being placed after
the delay slot, resulting in two nops.
While changing this code also guard the .set noreorder with .set push/pop
and use $zero as the cpsetup save register since we don't need to save $gp.

Reviewed By: jhb
Differential Revision: https://reviews.freebsd.org/D25025