Make sure build_* devtools scripts' default build directories always exist

Summary:
Both of these scripts assume the build directory already exists.
This patch makes sure it exists so the scripts do not fail otherwise.

Test Plan:

rm -rf build
./contrib/devtools/build_cmake.sh

rm -rf build
THREADS=8 ./contrib/devtools/build_autotools.sh

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5813