Fix default value for AR

arm-none-eabi-gcc --print-prog-name gcc-ar just prints gcc-ar, whereas
ar prints <prefix>/bin/../lib/gcc/arm-none-eabi/9.3.1/../../../../arm-none-eabi/bin/ar

Signed-off-by: Jagadeesh Ujja <jagadeesh.ujja@arm.com>
Change-Id: I511fc2d307e2a8cd5e223013f9dfcc391e3969e0