Build: Update Armclang version warning

Defect [SDCOMP-59788] has been fixed in Armclang 6.18.
Update Armclang version check to throw a warning only for Armclang
6.15~6.17.

Change-Id: I244120cd6c140873995af8898ad45976ddfa27f1
Signed-off-by: David Hu <david.hu@arm.com>
1 file changed