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pc, pc64: extend initial memory map for qemu multiboot data

qemu puts multiboot data after the end of the kernel image, so
to be able to KADDR() that memory early, we extend the initial
identity mapping by 16K. right now we just got lucky with
the pc kernel as it rounds the map to 4MB pages.

--- a/sys/src/9/pc/l.s

+++ b/sys/src/9/pc/l.s

@@ -186,6 +186,9 @@

 	MOVL	$PADDR(CPU0PTE), AX		/* first page of page table */

 	MOVL	$end-KZERO(SB), CX

+

+	ADDL	$(16*1024), CX			/* qemu puts multiboot data after the kernel */

+

 	ADDL	$(BY2XPG-1), CX

 	ANDL	$~(BY2XPG-1), CX		/* round to 4MB */

 	MOVL	CX, MemMin-KZERO(SB)		/* see memory.c */

--- a/sys/src/9/pc64/l.s

+++ b/sys/src/9/pc64/l.s

@@ -149,6 +149,9 @@

 	 */

 	ADDL	$PDO(KZERO), AX

 	MOVL	$end-KZERO(SB), CX

+

+	ADDL	$(16*1024), CX			/* qemu puts multiboot data after the kernel */

+

 	ADDL	$(PGLSZ(1)-1), CX

 	ANDL	$~(PGLSZ(1)-1), CX

 	MOVL	CX, MemMin-KZERO(SB)		/* see memory.c */