MFC r334505:
Don't export _end on arm64 and riscv.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Remove legacy brk and sbrk from RISC-V.

Discussed with: andrew
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5

libc: stop exporting curbrk and minbrk in the private namespace

They are not used anywhere else in the base system and are an internal
implementation detail that does not need to be exposed.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5728

Bring in initial libc and libstand support for RISC-V.

Reviewed by: andrew, emaste, kib
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5
Differential Revision: https://reviews.freebsd.org/D4943