Makefile.toolchain: Use portage env variable for host builds
Makefile.toolchain: Use portage env variable for host builds

When building the host tools with portage, we're not always building for
x86_64 (boards such as strongbad are ARM), so we need to use the Portage
environment variable to set the compiler prefix (HOST_CROSS_COMPILE).

BRANCH=none
BUG=b:246424843
TEST=emerge-strongbad chromeos-fpmcu-release-dartmonkey

Cq-Depend: chromium:3908089
Force-Relevant-Builds: all
Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I43cff8e01b3445cbce80c5d6fa45d7e7a36c4eda
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3905897
Reviewed-by: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec