x86inc: Avoid using eax/rax for storing the stack pointer

x86inc: Avoid using eax/rax for storing the stack pointer

When allocating stack space with an alignment requirement that is larger
than the current stack alignment we need to store a copy of the original
stack pointer in order to be able to restore it later.

If we chose to use another register for this purpose we should not pick
eax/rax since it can be overwritten as a return value.

Signed-off-by: Anton Khirnov <anton@khirnov.net>