util/xcompile: parallelize compiler checks

Speed up the execution of this script from ~6 seconds to ~1 on my
system.

There are some changes to its output, but they're actually _more_
correct: so far, architectures without compiler support kept compiler
options for architectures that ran successfully earlier.

Change-Id: I0532ea2178fbedb114a75cfd5ba39301e534e742
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/18262
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
1 file changed