break(2): allow on aarch64 and riscv

[brooks]

Summary

Now that the majority of upstream software using sbrk() has been fixed
and we've required manual linkage with libsbrk to access it, got ahead
and restore the undocumented break system call.

While this is arguably a regression, we've gotten most of the benefit of
not having sbrk and we're stuck with the core implementation on aarch64
for 32-bit support and both for linux emulation (technically not riscv
today, but we'll probably want linux emulation eventually.)

[brooks]

Spelling

[markj]

This is missing some pieces from D42853, namely a change to enable sbrk tests on riscv and arm64, and changes to the libc symbol list.

What is libsbrk? I haven't seen that before.

[brooks]

In D46953#1070216, @markj wrote:

This is missing some pieces from D42853, namely a change to enable sbrk tests on riscv and arm64, and changes to the libc symbol list.

Oops, will add.

What is libsbrk? I haven't seen that before.

See D43002. I reordered this commit in my branch and didn't update the message.