MIPS64: Implement sim-stack-size flag.

Some specific test requires larger sim stack size, like the mjsunit/harmony/proxies test.

The sim-stack-size flag was passed to the test already, but it was ignored by the MIPS64 simulator.

TEST=mjsunit/harmony/proxies
MIPS64: Align stack to 16 bytes for MIPS64.

The n64 ABI requires the stack is quadword aligned.

This resolves random mjsunit tests failures on HW.

TEST=mjsunit/debug-*, mjsunit/regress/regress-1853,
mjsunit/regress/regress-1853

BUG=