Avoid putting two safepoints in a row when doing self calls.

Because it's a loop it needs a safepoint, but there's already one at
the beginning of a function.
Restore exception handling on Windows-x86.

GCC decided to change its default stack alignment to 16 bytes, and now
we depend on aligning SAPs on the stack, the exception_handler_wrapper
assembly wrapper didn't maintain 16-byte stack alignment causing
problems. Fixing exception_handler_wrapper would be easy, but SBCL
only observes 4-byte alignment on x86oids, so it's better to convince
GCC with -mpreferred-stack-boundary=2 (and wait until we have the same
problem on Linux).