Add deprecation notice for WITH_PROFILE option

As discussed on freebsd-current [1] and freebsd-arch [2] and review
D30833, FreeBSD 14 will ship without the _p.a libraries built with -pg.
Both upstream and base system (in commit b762974cf4b9) Clang have been
modified to remove the special case for linking against these libraries.

Clang's -pg support and mcount() remain, so building with -pg can still
be used on code that the user builds; we just do not provide prebuilt
libraries compiled with -pg. A similar change is still needed for GCC.

[1] https://lists.freebsd.org/pipermail/freebsd-current/2020-January/075105.html
[2] https://lists.freebsd.org/archives/freebsd-arch/2021-June/000016.html

MFC after: 1 week
Sponsored by: The FreeBSD Foundation