[prefix] Make unlzma.S compatible with 386 class CPUs

Replace the bswap instruction with xchgb and roll and change the
module architecture from i486 to i386 to be consistent with the rest
of the project.

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

Author: jcapik

src/arch/x86/prefix/unlzma.S

@@ -45,7 +45,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 	.section ".note.GNU-stack", "", @progbits
 	.code32
-	.arch i486
+	.arch i386
 	.section ".prefix.lib", "ax", @progbits
 
 #ifdef CODE16
@@ -962,7 +962,9 @@ decompress:
 	ADDR32 lodsb	/* discard initial byte */
 	print_hex_byte %al
 	ADDR32 lodsl
-	bswapl	%eax
+	xchgb	%al, %ah
+	roll	$16, %eax
+	xchgb	%al, %ah
 	print_hex_dword %eax
 	print_character $('\n')
 	movl	%eax, rc_code(%ebp)