x86/cstart.S: initialize stack before using it

It seems the 32-bit initialization code uses the stack before actually
initializing it.

Probably the boot loader leaves a reasonable value in the stack pointer so
this issue has not been noticed before.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20200622162141.279716-2-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>