x86, boot: use BP_scratch in arch/x86/boot/compressed/head_*.S

Use the BP_scratch symbol from asm-offsets.h instead of hard-coding
the location.

[ Impact: cleanup ]
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

arch/x86/boot/compressed/head_32.S

@@ -53,7 +53,7 @@ ENTRY(startup_32)
  * data at 0x1e4 (defined as a scratch field) are used as the stack
  * for this calculation. Only 4 bytes are needed.
  */
-	leal (0x1e4+4)(%esi), %esp
+	leal (BP_scratch+4)(%esi), %esp
 	call 1f
 1:	popl %ebp
 	subl $1b, %ebp

arch/x86/boot/compressed/head_64.S

@@ -56,7 +56,7 @@ ENTRY(startup_32)
  * data at 0x1e4 (defined as a scratch field) are used as the stack
  * for this calculation. Only 4 bytes are needed.
  */
-	leal	(0x1e4+4)(%esi), %esp
+	leal	(BP_scratch+4)(%esi), %esp
 	call	1f
 1:	popl	%ebp
 	subl	$1b, %ebp