rtld_malloc: add __crt_aligned_alloc_offset()

It is modelled after aligned_alloc(3). Most importantly, to free the
allocation, __crt_free() can be used. Additionally, caller may specify
offset into the aligned allocation, so that we return offset-ed from
alignment pointer.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D41150