- consolidate target_record_sp_limit and target_get_sp_limit functions
for aarch64, powerpc, arm-ios and openbsd as there are all without
segmented stacks (no need to duplicate functions).
- rename __load_self function to rust_load_self
- use a mutex inner load_self() as underline implementation is not thread-safe
This commit merges the `rustrt` crate into `std`, undoing part of the
facade. This merger continues the paring down of the runtime system.
Code relying on the public API of `rustrt` will break; some of this API
is now available through `std::rt`, but is likely to change and/or be
removed very soon.
[breaking-change]
