Fix: symbols/bionic/lib64/libc.so is the wrong variant

The new module type bionic_mountpoint wasn't mutated by the sanitizer.
As a result, it has been taking non-sanitized symbol libraries even for
sanitized builds. Fixing the issue by making the module type to
implement the cc.Sanitizeable interface so that it can be mutated by the
sanitizer.

Bug: 124469750
Test: SANITIZE_TARGET=hwaddress m
Inspect Android-<target>.mk and check that LOCAL_SOONG_UNSTRIPPED_BINARY
for libc.mountpoint module is pointing to a hwasan variant of libc.so

Change-Id: I10c863c0dbd361463648a4b7d897a4f88a9c85cb