[X86] Don't clobber EBX in stackprobes

On X86, the stackprobe emission code chooses the `R11D` register, which
is illegal on i686.  This ends up wrapping around to `EBX`, which does
not get properly callee-saved within the stack probing prologue,
clobbering the register for the callers.

We fix this by explicitly using `EAX` as the stack probe register.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D109203