Currently, the final check-msan/scuda/... step is controlled by SKIP_MSAN/...
environment variables, set based on buildbot config. However, these variables
have a default value of 1 set in the bash script, making the tests never
On the other hand, the other two check-msan/... steps are executed iff
ARCH == x86_64, skipping the tests on all other architectures.
Let's nuke the SKIP_* variables, and make all check-msan/... steps
conditional on the ARCH, as determined by the bash script, but extend
it to recognize other architectures that support msan (and other sanitizers).
Also make tsan likewise conditional (it's not supported eg. on i386).