ARM: 9039/1: assembler: generalize byte swapping macro into rev_l

Take the 4 instruction byte swapping sequence from the decompressor's
head.S, and turn it into a rev_l GAS macro for general use. While
at it, make it use the 'rev' instruction when compiling for v6 or
later.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>