arm: use canonical sub mnemonic

Building some arm boards with older binutils may produce errors like this:

---8<---
crt0.S: Assembler messages:
crt0.S:70: Error: register expected, not '#(184)' -- `sub sp,#(184)'
--->8---

Use canonical version of the subtract mnemonic to avoid those issues.

Reported-by: Alexey Smishlayev <alexey@xtech2.lv>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>