[ARM] 5404/1: Fix condition in arm_elf_read_implies_exec() to set READ_IMPLIES_EXEC

READ_IMPLIES_EXEC must be set when:
o binary _is_ an executable stack (i.e. not EXSTACK_DISABLE_X)
o processor architecture is _under_ ARMv6 (XN bit is supported from ARMv6)

Signed-off-by: Makito SHIOKAWA <lkhmkt@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>