scudo: Only static_assert for compressed LSB format with clang.

It looks like there's some old version of gcc that doesn't like this
static_assert (I couldn't reproduce the issue with gcc 8, 9 or 10).
Work around the issue by only checking the static_assert under clang,
which should provide sufficient coverage.

Should hopefully fix this buildbot:
https://lab.llvm.org/buildbot/#/builders/112/builds/5356