[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/romprefix.S

@@ -591,8 +591,11 @@ exec:	/* Set %ds = %cs */
 	pushl	$main
 	pushw	%cs
 	call	prot_call
-	/* No need to clean up stack; we are about to reload %ss:sp */
-	
+	popl	%ecx /* discard */
+
+	/* Uninstall gPXE */
+	call	uninstall
+
 	/* Restore BIOS stack */
 	movw	%dx, %ss
 	movw	%bp, %sp