libc/aarch64: Copy the trivial getcontextx

This will be used as the base of a non-trivial implementation.

Reviewed by: kib
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44254

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

libc: Remove _get_tp() and _set_tp().

Their uses have been replaced by _tcb_get() and _tcb_set() from
<machine/tls.h>.

Reviewed by: kib, jrtc27
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33354

libc: add _get_tp() private function

which returns pointer to tcb

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D29623

Disable support for compat syscalls on arm64. These symbols were never
shipped since arm64 exists only on 11+.

Submitted by: brooks
Reviewed by: emaste, imp

Add the definitions of __infinity and __nan.

Add support for makecontext. This supports up to 8 arguments as this
simplifies the code, these can be passed in registers.

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation

Implement fpgetmask, it's needed by Python.

Approved by: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation

Implement fpsetmask. Some third-party software makes use of it, for example
perl.

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation

Add fabs() to arm64 libc

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D2532

Add the start of libc and libstand for arm64. Not all of the machine
dependent functions have been implemented, but this is enough for world.

Differential Revision: https://reviews.freebsd.org/D2132
Reviewed by: emaste
Sponsored by: The FreeBSD Foundation