curve25519: eliminate unnecessary "callee save" prologues

The assembly implementations of ladderstep and mul contain register
save prologues that are unnecessary in Go because there are no callee
save registers in the Go ABI. Remove these prologues, update all SP
offsets, and reduce the frame size accordingly.

The SP offsets were updated with:

python -c 'import sys, re; sys.stdout.write(re.sub(r"(\d+)\(SP\)", lambda m: "%d(SP)" % (int(m.group(1))-YYY), sys.stdin.read()))'

where YYY was 64 for mul_amd64.s and 56 for ladderstep_amd64.s.

Change-Id: I728948809f479b1c061cc65167dadad651efab31
Reviewed-on: https://go-review.googlesource.com/31580
Reviewed-by: Adam Langley <[email protected]>
Reviewed-by: Minux Ma <[email protected]>

Author: aclements