buildgcc: Fix colors for dash

The previous fix broke buildgcc colors on MacOS X.
This uses an encoding that should be more universal.

Change-Id: I31ac6090ffb7c04784cf6566823652f229aebbb5
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/361
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>