xcompile: test if gcc is really available

Just because an 'as' with a certain prefix is available does not guarantee
that a 'gcc' with the same prefix is available as well.

Without a check detect_compiler_runtime() would try to execute an
unavailable binary and print something like this:
.../xcompile: line 218: arm-linux-gnueabi-gcc: command not found

Change-Id: Icbadfeb2860152f7cf7696a9122521d0d881f3aa
Signed-off-by: Stefan Tauner <stefan.tauner@gmx.at>
Reviewed-on: https://review.coreboot.org/14563
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
1 file changed