[sanitizer] Handle Gentoo's libstdc++ path

On Gentoo, libc++ is indeed in /usr/include/c++/*, but libstdc++ is at
e.g. /usr/lib/gcc/x86_64-pc-linux-gnu/14/include/g++-v14.

Use '/include/g++' as it should be unique enough. Note that the omission of
a trailing slash is intentional to match g++-*.

See https://github.com/llvm/llvm-project/pull/78534#issuecomment-1904145839.

Reviewed by: mgorny
Closes: https://github.com/llvm/llvm-project/pull/79264



Signed-off-by: Sam James <sam@gentoo.org>