libc: stop exporting curbrk and minbrk in the private namespace

[emaste]

Summary

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

If there's a concern we could leave them on existing archs and remove them from arm64 and riscv at least.

[kib]

How did you checked that the symbols are not used ?

Did you verified the import list of all base libs and binaries ?

[emaste]

In D5728#122338, @kib wrote:

How did you checked that the symbols are not used ?

Did you verified the import list of all base libs and binaries ?

I checked the libs and binaries on my amd64 desktop, and did a quick inspection of source changes since FreeBSD 7.