linux(4): Remove sys/cdefs.h inclusion where it's not needed due to 685dc743

sys: Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

linux(4): Cleanup includes under amd64/linux

Cleanup unneeded includes, sort the rest according to style(9).
No functional changes.

MFC after: 2 weeks

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

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

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linux(4): Better naming for ucontext field of struct rt_sigframe

To reduce sendsig code difference and to avoid confusing me,
rename sf_sc to sf_uc to match the content.

MFC after: 2 weeks

linux(4): Move sigframe definitions to separate headers

The signal trampoine-related definitions are used only in the MD part
of code, wherefore moved from everywhere used linux.h to separate MD
hea

linux(4): Move sigframe definitions to separate headers

The signal trampoine-related definitions are used only in the MD part
of code, wherefore moved from everywhere used linux.h to separate MD
headers.

MFC after: 2 weeks

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linux(4): Cleanup signal trampolines

This is the first stage of a signal trampolines refactoring.

From trampolines retired emulation of the 'call' instruction, which is
replaced by direct call of a

linux(4): Cleanup signal trampolines

This is the first stage of a signal trampolines refactoring.

From trampolines retired emulation of the 'call' instruction, which is
replaced by direct call of a signal handler. The signal handler address
is in the register.

The previous trampoline implemenatation used semi-Linux-way to call
a signal handler via the 'jmp' instruction. Wherefore the trampoline
emulated a 'call' instruction to into the stack the return address for
signal handler's 'ret' instruction. Wherefore handmade DWARD annotations
was used.

While here rephrased and removed excessive comments.

MFC after: 2 weeks

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Catch up with HEAD (r280229-r284686).

MFH: r282615-r283655

Sponsored by: The FreeBSD Foundation

Add preliminary support for x86-64 Linux binaries.

Differential Revision: https://reviews.freebsd.org/D1076