build: prefer `gcc-ar` over `ar`

The `gcc-ar` wrapper exists to make it easier to support LTO on some
versions of GCC. The two commands are compatible, accepting exactly the
same arguments, so this change moves us to `gcc-ar` to ensure that we
are configuring LTO correctly.

Change-Id: I24a4cfaad29d35b09f847299081f83ca9b41aa8a
Signed-off-by: Chris Kay <chris.kay@arm.com>
3 files changed