[Patch V2] mips: minior fix

[Huang Pei]

All three patched is tested on debian glibc-2.28, no regresssion
found.

here is the output of 'LD_DEBUG=statistics data' with Patch 1:

.....................................................................
depaulose@Board-3A3000:glibc-2.28$ LD_DEBUG=statistics date
	441:
	441:     runtime linker statistics:
	441:       total startup time in dynamic loader: 297131 cycles
	441:       time needed for relocation: 150697 cycles (50.7%)
	441:       number of relocations: 101
	441:       number of relocations from cache: 0
	441:       number of relative relocations: 0
	441:       time needed to load objects: 84028 cycles (28.2%)
Sat 14 Nov 2020 06:33:18 AM UTC
.....................................................................

Patch 2 make the sp follow N32/N64 ABI, however the root cause is the
reg s0 spill, which is unnecessary