gem5/gem5-aarch64-native: don't build gem5 three times

The recipe currently builds gem5 binaries in optimised, fast, and
debug mode.  There is no need to build three at once (and triples the
build time), so simply build the optimised variant by default (as recommended
by upstream) whilst letting the variant be altered by setting
GEM5_BUILD_VARIANT.

Change-Id: I23baeee438d1ff08d1e2a54282141b55500f2ac1
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>