[build] Ensure stacks are at edges of used memory

Under VMware, memory above %esp somehow gets overwritten, leading to
corrupt internal data structures and unpredictable behaviour.

Ensure that the protected-mode stack is at the top of our used memory,
and that the real-mode stack is at the bottom.  This keeps all of the
important data structures nicely contained within a region of memory
that is exclusively ours.

Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>

callback.S

@@ -218,7 +218,7 @@ rm_idtr:
 	.size	rm_idtr, . - rm_idtr
 
 	/* Real-mode stack */
-	.section ".bss16", "aw", @nobits
+	.section ".stack16", "aw", @nobits
 	.align	8
 _rmstack:
 	.space	8192

script.lds

@@ -16,6 +16,8 @@ SECTIONS {
 	/* Real-mode uninitialised data section */
 	.bss16 ( NOLOAD ) : {
 		_bss16 = .;
+		*(.stack16)
+		*(.stack16.*)
 		*(.bss16)
 		*(.bss16.*)
 		_ebss16 = .;
@@ -48,6 +50,9 @@ SECTIONS {
 		_bss = .;
 		*(.bss)
 		*(.bss.*)
+		*(COMMON)
+		*(.stack)
+		*(.stack.*)
 		_ebss = .;
 	}
 	_bss_len = ABSOLUTE ( _ebss ) - ABSOLUTE ( _bss );

startup.S

@@ -109,7 +109,7 @@ idt:
 	.size	idt, . - idt
 
 	/* Stack */
-	.section ".bss", "aw", @nobits
+	.section ".stack", "aw", @nobits
 	.align	8
 _stack:
 	.space	( 256 * 1024 )