Implement new low-level signal trampoline

See merge request !460

Fix thread spawning on aarch64

See merge request !463

* Correctly align the stack
* Fix argument order of the ldp instructions
* Remove unnecessary ldr x5 instruction

Add tests for further sys/resource.h constants

See merge request !462

Add stub for SIOCATMARK command of ioctl.

See merge request !461

Fix posix prio process (same as MR458 with fixed commits)

See merge request !459

Use standard casing and mangling for internal errno

See merge request redox-os/relibc!457

Add Crypt functions

See merge request redox-os/relibc!453

Use Cell representation for errno

See merge request redox-os/relibc!456

Panic when getpagesize() cannot succeed as specified

See merge request redox-os/relibc!455

Update rust-toolchain.toml

See merge request redox-os/relibc!452

This matches the version actually used by Redox OS. It also supports the
sparse index, which prevents having to clone the crates.io registry git
repo when rust-analyzer tries to run cargo metadata on relibc.

Scheme fmt

See merge request redox-os/relibc!451

Add forkpty, ptsname and posix_openpt

See merge request redox-os/relibc!450