ARM: 6502/1: Thumb-2: Fix CONFIG_THUMB2_KERNEL breakage in compressed/head.S

Some instruction operand combinations are used here which are nor
permitted in Thumb-2.

In particular, most uses of pc as an operand are disallowed in
Thumb-2, and deprecated in ARM from ARMv7 onwards.

The modified code introduced by this patch should be compatible
with all architecture versions >= v3, with or without
CONFIG_THUMB2_KERNEL.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Dave Martin <dave.martin@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>