xtensa: add support for KASAN

Cover kernel addresses above 0x90000000 by the shadow map. Enable
HAVE_ARCH_KASAN when MMU is enabled. Provide kasan_early_init that fills
shadow map with writable copies of kasan_zero_page. Call
kasan_early_init right after mmu initialization in the setup_arch.
Provide kasan_init that allocates proper shadow map pages from the
memblock and puts these pages into the shadow map for addresses from
VMALLOC area to the end of KSEG. Call kasan_init right after memblock
initialization. Don't use KASAN for the boot code, MMU and KASAN
initialization and page fault handler. Make kernel stack size 4 times
larger when KASAN is enabled to avoid stack overflows.
GCC 7.3, 8 or newer is required to build the xtensa kernel with KASAN.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/lib/memset.S

@@ -31,7 +31,8 @@
  */
 
 .text
-ENTRY(memset)
+ENTRY(__memset)
+WEAK(memset)
 
 	entry	sp, 16		# minimal stack frame
 	# a2/ dst, a3/ c, a4/ length
@@ -140,7 +141,7 @@ EX(10f) s8i	a3, a5, 0
 .Lbytesetdone:
 	retw
 
-ENDPROC(memset)
+ENDPROC(__memset)
 
 	.section .fixup, "ax"
 	.align	4