toolchain_util: Add arm64-usr support
toolchain_util: Add arm64-usr support

Adds CROSS_PROFILES, BOARD_CHOSTS, and BOARD_PROFILES definitions to support a
generic arm64-usr board.

get_portage_arch() is updated to convert aarch64 correctly.

Signed-off-by: Geoff Levand <c3918f39022407dee6a0056ceca9b1aa80e8e4ce@infradead.org>

mjg59/scripts,marineam/coreos-scripts,sigma/coreos-scripts,nicescale/scripts,fivethreeo/scripts,endocode/scripts,fivethreeo/scripts,gtank/scripts,BugRoger/scripts,smilart/scripts,fivethreeo/scripts,endocode/scripts,nicescale/scripts,andrejro/coreos--scripts,coreos/scripts,mjg59/scripts,trnubo/scripts,endocode/scripts,andrejro/coreos--scripts,marineam/coreos-scripts,zhang0137/scripts,andrejro/coreos--scripts,gtank/scripts,glevand/coreos--scripts,mjg59/scripts,smilart/scripts,zhang0137/scripts,trnubo/scripts,BugRoger/scripts,marineam/coreos-scripts,trnubo/scripts,glevand/coreos--scripts,nicescale/scripts,smilart/scripts,zhang0137/scripts,sigma/coreos-scripts,coreos/scripts,sigma/coreos-scripts,BugRoger/scripts,gtank/scripts