[NFCI] Change llvm-ar '-format' to be '--format'.

Upcoming versions of the toolchain (at least when I was locally testing)
drop support for the single-dashed argument type. Fixing this ahead of
time as to hopefully avoid the toolchain-release-blocking bug in future
:).

Bug: N/A
Test: Build the platform.
Change-Id: Ia2c6dcb19556f1979f9a6ecfd3a0ec58fb73e36e