imgact_elf: Export __elfN(parse_notes)

This is useful to check if a note is present and contains an expected
value, e.g. to read NT_GNU_PROPERTY_TYPE_0 on arm64 to see if we should
enable BTI.

Reviewed by: kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42439