[MSan] Adding support for 32-bit platforms

[k-kashapov]

As stated in this issue LLVM MSan does not support the 32-bit architectures. The reason is that shadow memory is considered too big for these platforms.

However, we have been successfully using MSan on embedded 32-bit platforms. We would like to implement VaArgHelper for these architectures. We will provide the patch later.

Do you consider it possible adding such feature to the upstream?

cc @vitalybuka @EugeneZelenko

[vitalybuka]

It should be fine if it's not complicate general instrumentation.
WDYT? @eugenis