MIPS64: Fix Trampoline slot size and c++ sign-extend error.

Trampoline slot size is 8 * isntrSize in mips64.

int32_t a;
int64_t b;
b |= (a & 0xffff) << 16.
The right part will sign-extern to int64_t which maybe not want.

TEST: mjsunit/switch.