[libc] Fix typo in longjmp()

Commit 8ab4b00 ("[libc] Rewrite setjmp() and longjmp()") introduced a
regression in which the saved values of %ebx, %esi, and %edi were all
accidentally restored into %esp.  The result is that the second and
subsequent returns from setjmp() would effectively corrupt %ebx, %esi,
%edi, and the stack pointer %esp.

Use of setjmp() and longjmp() is generally discouraged: our only use
occurs as part of the implementation of PXENV_RESTART_TFTP, since the
PXE API effectively mandates its use here.  The call to setjmp()
occurs at the start of pxe_start_nbp(), where there are almost
certainly no values held in %ebx, %esi, or %edi.  The corruption of
these registers therefore had no visible effect on program execution.
The corruption of %esp would have been visible on return from
pxe_start_nbp(), but there are no known PXE NBPs which first call
PXENV_RESTART_TFTP and subsequently attempt to return to the PXE base
code.  The effect on program execution was therefore similar to that
of moving the stack to a pseudo-random location in the 32-bit address
space; this will often allow execution to complete successfully since
there is a high chance that the pseudo-random location will be unused.
The regression therefore went undetected for around one month.

Fix by restoring the correct registers from the saved jmp_buf
structure.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/core/setjmp.S

 1:	/* Restore stack pointer */
 	movl	env_stack(%edx), %esp
 	/* Restore other registers */
-	movl	env_ebx(%edx), %esp
-	movl	env_esi(%edx), %esp
-	movl	env_edi(%edx), %esp
+	movl	env_ebx(%edx), %ebx
+	movl	env_esi(%edx), %esi
+	movl	env_edi(%edx), %edi
 	movl	env_ebp(%edx), %ebp
 	/* Replace return address on the new stack */
 	popl	%ecx	/* discard */