libsanitizer: merge from upstream (c425db2eb558c263)

The following patch is result of libsanitizer/merge.sh
from c425db2eb558c263 (yesterday evening).

Bootstrapped/regtested on x86_64-linux and i686-linux (together with
the follow-up 3 patches I'm about to post).

BTW, seems upstream has added riscv64 support for I think lsan/tsan,
so if anyone is willing to try it there, it would be a matter of
copying e.g. the s390*-*-linux* libsanitizer/configure.tgt entry
to riscv64-*-linux* with the obvious s/s390x/riscv64/ change in it.