BACKPORT: 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>

(cherry picked from commit 6468e898c67b905ec0f95d9678929135bcaf7f67)
(resolved conflict due to different patch context, caused by missing
 other macros in assembler.h)
Bug: 178411248
Change-Id: I8433e97d2880f75cace215f1a8daadec7f29929c
Signed-off-by: Eric Biggers <ebiggers@google.com>