adjust mips crt_arch entry point asm to avoid assembler bugs

adjust mips crt_arch entry point asm to avoid assembler bugs

apparently the .gpword directive does not work reliably with local
text labels; values produced were offset by 64k from the correct
value, resulting in incorrect computation of the got pointer at
runtime. instead, use an external label so that the assembler does not
munge the relocation; the linker will then get it right.

commit 6fef8cafbd0f6f185897bc87feb1ff66e2e204e1 exposed this issue by
removing the old, non-PIE-compatible handwritten crt1.s, which was not
affected. presumably mips PIE executables (using Scrt1.o produced from
crt_arch.h) were already affected at the time.