build/roms: Build 32-bit crossgcc for AArch64 as well

This re-applies commit a69855f7

 ("Build 32-bit crossgcc for AArch64
as well") which was inexplicably reverted along with unrelated changes.
Mention in a comment that building crossgcc-arm is necessary for
AArch64.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>