sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

linux(4): Retire handmade DWARF annotations from signal trampolines

The Linux exports __kernel_sigreturn and __kernel_rt_sigreturn from the
vdso. Modern glibc's sigaction sets the sa_restorer field of sigaction
to the corresponding vdso __sigreturn, and sets the SA_RESTORER.
Our signal trampolines uses the FreeBSD-way to call a signal handler,
so does not use the sigaction's sa_restorer.

However, as glibc's runtime linker depends on the existment of the vdso
__sigreturn symbols, for all Linuxulators was added separate trampolines
named __sigcode with DWARF anotations and left separate __sigreturn
methods, which are exported.

MFC after: 2 weeks

linux(4): Implement vdso getcpu for x86.

This is modeled after f2395455 (by kib@).

MFC after: 2 weeks

linux(4); Almost complete the vDSO.

The vDSO (virtual dynamic shared object) is a small shared library that the
kernel maps R/O into the address space of all Linux processes on image
activation. The vDSO is a fully formed ELF image, shared by all processes
with the same ABI, has no process private data.

The primary purpose of the vDSO:
- non-executable stack, signal trampolines not copied to the stack;
- signal trampolines unwind, mandatory for the NPTL;
- to avoid contex-switch overhead frequently used system calls can be
implemented in the vDSO: for now gettimeofday, clock_gettime.

The first two have been implemented, so add the implementation of system
calls.

System calls implemenation based on a native timekeeping code with some
limitations:
- ifunc can't be used, as vDSO r/o mapped to the process VA and rtld
can't relocate symbols;
- reading HPET memory is not implemented for now (TODO).

In case on any error vDSO system calls fallback to the kernel system
calls. For unimplemented vDSO system calls added prototypes which call
corresponding kernel system call.

Tested by: trasz (arm64)
Differential revision: https://reviews.freebsd.org/D30900
MFC after: 2 weeks

Put linux_platform into the vdso to avoid copying it onto the stack at
every exec.

Differential Revision: https://reviews.freebsd.org/D1062
Reviewed by: trasz

Implement vdso - virtual dynamic shared object. Through vdso Linux
exposes functions from kernel with proper DWARF CFI information so that
it becomes easier to unwind through them.
Using vdso is a mandatory for a thread cancelation && cleanup
on a modern glibc.

Differential Revision: https://reviews.freebsd.org/D1060