Include ABI note tag in shared libraries.

Split the ELF feature note into a separate file that is linked into
*crt1.o the same as crtbrand.S was before. crtbrand.o is now linked
into crti.o on all platforms in addition to *crt1.o.

Reviewed by: kib
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25304