build system: only query the compiler runtime's location once

No need to execute the compiler to figure this out once for each
source file (or so).

Change-Id: I56bf084f1217b96748296931617e9233f21183d5
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/10294
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
1 file changed