Build: Fix GNU "specs" command line argument prefix

GNU 12.2.Rel1 only parse this argument with one "-".
Every ARM GNU manuals contains "-specs" instead of "--specs".

Signed-off-by: Dávid Házi <david.hazi@arm.com>
Change-Id: I206930d65ac0f7d47fb693e9ec8dc8cbc884dffc
1 file changed