[zircon][gn] Use AS_NEEDED for libc++.so

This avoids the extra DT_NEEDED entry even in cases where libc++.so
isn't needed. This is also consistent with Clang driver implementation
which uses --as-needed around libc++.so.

Bug: ZX-4099
Change-Id: I76475896b7ea2c825a97f28b7b09e53228b468c6