[klibc] mips/mips64: simplify crt0 code

Various simplifications and adjustments to the MIPS crt0 files:
- Use NESTED(__start, 0, ra) - this has no effect on the code, but is
  arguably more correct since we have no real stack frame.
- Don't allocate extra stack space. We need none on 64-bit, and only 16
  bytes on 32-bit.
- Align the stack pointer in the (unlikely) event it is misaligned.
- Don't load the gp register - it is useless in non-PIC code.
- Use jal in 64-bit code.
- Crash if __libc_start returns (teq should causes a trap exception).

Signed-off-by: James Cowgill <james.cowgill@mips.com>
Link: https://www.zytor.com/pipermail/klibc/2018-March/003982.html
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
2 files changed