Make asan more closely match clang behavior.

Always link libm with asan. Hasn't been a problem before because ASAN
was only ever used when libc++ was as well, which already links libm.

Pass --no-as-needed for host modules.

These aren't needed for the target builds because the target uses the
shared RTL.

Change-Id: I5d6a3c2dd557b5231be2b7711df6438693753a88