toolchain-external: factorize regular expressions

Based on a suggestion from Peter, this commit factorizes the logic and
regular expressions that are used to find the sysroot and libdir for a
given compiler. It reduces a bit the duplication of code, and
centralizes the most bizarre part of this logic in one place.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>