[pcbios] Guard against register corruption in INT 15,e820 implementations

Someone at Dell must have a full-time job designing ways to screw up
implementations of INT 15,e820.  This latest gem is courtesy of a Dell
Xanadu system, which arbitrarily decides to obliterate the contents of
%esi.

Preserve %esi, %edi and %ebp across calls to INT 15,e820, in case
someone tries a variation on this trick in future.

src/arch/i386/firmware/pcbios/e820mangler.S

 	pushl	%ebx
 	pushl	%ecx
 	pushl	%edx
+	pushl	%esi	/* Some implementations corrupt %esi, so we	*/
+	pushl	%edi	/* preserve %esi, %edi and %ebp to be paranoid	*/
+	pushl	%ebp
 	pushw	%es
-	pushw	%di
 	pushw	%ds
 	popw	%es
 	movw	$underlying_e820_cache, %di
 	stc
 	pushfw
 	lcall	*%cs:int15_vector
-	popw	%di
 	popw	%es
+	popl	%ebp
+	popl	%edi
+	popl	%esi
 	/* Check for error return from underlying e820 call */
 	jc	2f /* CF set: error */
 	cmpl	$SMAP, %eax

src/arch/i386/firmware/pcbios/memmap.c

 	memset ( &e820buf, 0, sizeof ( e820buf ) );
 
 	do {
+		/* Some BIOSes corrupt %esi for fun. Guard against
+		 * this by telling gcc that all non-output registers
+		 * may be corrupted.
+		 */
 		__asm__ __volatile__ ( REAL_CODE ( "stc\n\t"
 						   "int $0x15\n\t"
 						   "pushfw\n\t"
 					 "D" ( __from_data16 ( &e820buf ) ),
 					 "c" ( sizeof ( e820buf ) ),
 					 "d" ( SMAP )
-				       : "memory" );
+				       : "esi", "memory" );
 
 		if ( smap != SMAP ) {
 			DBG ( "INT 15,e820 failed SMAP signature check\n" );