build: use generic name for arm_algorithm vars

build: use generic name for arm_algorithm vars

This makes the code a bit easier to read as arm_algorithm can
refer to other arch's, not just armv4_5.

Change-Id: I78c99d40f34cda04e06f2daee75b48ff40a1d23d
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/613
Tested-by: jenkins
Reviewed-by: Aurelien Jacobs <aurel@gnuage.org>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>