Support 16-byte aligned stack on 32-bit Windows.

Support 16-byte aligned stack on 32-bit Windows.

On Windows x86-32, the ABI only guarantees the stack to be 4-byte
aligned. We therefore need the stack pointer to be explicitly
aligned when using vectors. This demands using a frame pointer (to
access function arguments). Also, we had to change accessing spilled
variables from the stack pointer instead of the frame pointer so they
are also aligned. This change does not affect PNaCl. Projects using
the Microsoft ABI should define SUBZERO_USE_MICROSOFT_ABI.

BUG=swiftshader:29

Change-Id: I186ce9435244d6fa9494ec514a91122b6be130b3
Reviewed-on: https://chromium-review.googlesource.com/427348
Reviewed-by: Jim Stichnoth <stichnot@chromium.org>