MIPS64 has 32 general purpose 64-bit integer registers (R0-R31), 32
64-bit floating point registers (F0-F31). Instructions are fixed-width,
and are 32-bit wide. Instructions are all in standard 1-, 2-, 3-operand
forms.

MIPS64-specific relocations are added. For this reason, test data of
cmd/newlink are regenerated.

No other changes are made to portable structures.

Branch delay slots are current filled with NOP instructions. The function
for instruction scheduling (try to fill the delay slot with a useful
instruction) is implemented but disabled for now.

Change-Id: Ic364999c7a33245260c1381fc26a2fa8972d38b3
Reviewed-on: https://go-review.googlesource.com/14442Reviewed-by: Minux Ma <minux@golang.org>