feat(hermit): add stabilized interface

HermitOS is a unikernel and its interface to the kernel is provided
by https://crates.io/crates/hermit-abi. In the meantime parts of the
interface is stabilized and we want to integrated it into libc.
Unstable version will be still provided by hermit-abi.

Co-authored-by: Martin Kröning <martin.kroening@eonerc.rwth-aachen.de>
Signed-off-by: Martin Kröning <martin.kroening@eonerc.rwth-aachen.de>

[Fix conflicts - Trevor]
(backport <rust-lang#3724>)
(cherry picked from commit 03feff5)
Signed-off-by: Trevor Gross <tmgross@umich.edu>