Avoid the 'bx' instruction on CPUs that have no support for Thumb and
thus do not implement this instruction by moving the generation of this
opcode to a separate function that selects between:

	bx	reg

and

	mov	pc, reg

according to the capabilities of the CPU.

Fixes: 39c13c20 ("arm: eBPF JIT compiler")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>