Add core support for ARMv6/v7 big-endian

Starting with ARMv6, the CPUs support the BE-8 variant of big-endian
(byte-invariant). This patch adds the core support:

- setting of the BE-8 mode via the CPSR.E register for both kernel and
  user threads
- big-endian page table walking
- REV used to rotate instructions read from memory during fault
  processing as they are still little-endian format
- Kconfig and Makefile support for BE-8. The --be8 option must be passed
  to the final linking stage to convert the instructions to
  little-endian

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm/kernel/entry-armv.S

@@ -482,6 +482,9 @@ __und_usr:
 	subeq	r4, r2, #4			@ ARM instr at LR - 4
 	subne	r4, r2, #2			@ Thumb instr at LR - 2
 1:	ldreqt	r0, [r4]
+#ifdef CONFIG_CPU_ENDIAN_BE8
+	reveq	r0, r0				@ little endian instruction
+#endif
 	beq	call_fpe
 	@ Thumb instruction
 #if __LINUX_ARM_ARCH__ >= 7