mips o32: fix build

mips o32: fix build

Commit 917c2ccf3a67 "Refactor stack pointers" moved mips_REG_* macros
from linux/mips/arch_regs.h to linux/mips/arch_regs.c because these
macros are no longer used outside syscall.c or files included by
syscall.c, but this caused a build regression on mips o32 because
decode_syscall_subcall() uses mips_REG_SP prior to its definition.

* syscall.c (decode_syscall_subcall): Move ...
* linux/mips/get_syscall_args.c: ... here.
* NEWS: Mention this fix.

Reported-by: Baruch Siach <baruch@tkos.co.il>
Fixes: v4.26~61 "Refactor stack pointers"