Build: Make Armclang 6.15~6.17 version check as an error

Commit a135228 converted Armclang version check against [SDCOMP-59788]
to an warning as users might rely on 6.15~6.17 for new features.

As Armclang 6.18 is released and fixes [SDCOMP-59788], users shall use
Armclang 6.18 instead.
Make the check as a fatal error again as the warning can be ignored or
missed easily.

Change-Id: Ib934164b1fbcfb25344c33df34aa94e558e5f9bd
Signed-off-by: David Hu <david.hu@arm.com>
1 file changed