[i386] Free allocated base memory on exit, if possible

Code paths that automatically allocate memory from the FBMS at 40:13
should also free it, if possible.

Freeing this memory will not be possible if either

  1. The FBMS has been modified since our allocation, or

  2. We have not been able to unhook one or more BIOS interrupt vectors.

src/arch/i386/prefix/dskprefix.S

@@ -353,6 +353,7 @@ msg1end:
 	.word 0xAA55
 
 start_runtime:
+	/* Install gPXE */
 	call	install
 
 	/* Set up real-mode stack */
@@ -368,7 +369,10 @@ start_runtime:
 	pushl	$main
 	pushw	%cs
 	call	prot_call
-	popl	%eax /* discard */
+	popl	%ecx /* discard */
+
+	/* Uninstall gPXE */
+	call	uninstall
 
 	/* Boot next device */
 	int $0x18