build system: move compiler runtime determination to xcompile

Instead of fetching libgcc's location and required compiler flags on every
individual build, do it once in xcompile.

Change-Id: Ie5832fcb21710c4cf381ba475589d42ce0235f96
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/10425
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <edward.ocallaghan@koparo.com>
2 files changed