kernel, arm ucontext: ARM DBG=-g run fixes

kernel:
	. modules can be as big as the space (8MB) between them
	  instead of 4MB; memory is slightly bigger with DBG=-g

arm ucontext:
	. r4 is clobbered by the restore function, as it's
	  used as a scratch register, causing problems for the
	  DBG=-g build
	. r1-r3 are safe for scratch registers, as they are
	  caller-save, so use r3 instead; and don't bother
	  restoring r1-r3, but preserve r4

vfs:
	. improve TLL pointer sanity check a bit

Change-Id: I0e3cfc367fdc14477e40d04b5e044f288ca4cc7d

kernel/arch/earm/pre_init.c

@@ -105,7 +105,6 @@ int overlaps(multiboot_module_t *mod, int n, int cmp_mod)
 #define MB_MODS_BASE  0x82000000
 #define MB_PARAM_MOD  0x88000000
 #define MB_MODS_ALIGN 0x00800000 /* 8 MB */
-#define MB_MODS_SIZE  0x00400000 /* 4 MB */
 #define MB_MMAP_START 0x80000000
 #define MB_MMAP_SIZE  0x10000000 /* 256 MB */
 
@@ -123,7 +122,8 @@ void setup_mbi(multiboot_info_t *mbi)
 	int i;
 	for (i = 0; i < MB_MODS_NR; ++i) {
 	    mb_modlist[i].mod_start = MB_MODS_BASE + i * MB_MODS_ALIGN;
-	    mb_modlist[i].mod_end = mb_modlist[i].mod_start + MB_MODS_SIZE ;
+	    mb_modlist[i].mod_end = mb_modlist[i].mod_start + MB_MODS_ALIGN
+		- ARM_PAGE_SIZE;
 	    mb_modlist[i].cmdline = 0;
 	}