When returning from a Linux signal use the Linux sigframe to find the
register values to restore.
Remove the FreeBSD ucontext from the stack as it's now unneeded.
PR: 270250
Reviewed by: dchagin, emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42360
(cherry picked from commit 070a4ff82a34652d533f9315ae9ad0aa8f1fdeb2)
The implemenation differs from others Linuxulators.
For unwinders Linux ucontext_t is stored, however native machine context
is used to store/restore process state to avoid code duplication.
As DWARF Aarch64 does not define a register number for PC and provides no
direct way to encode the PC of the previous frame, CFI cannot describe a
signal trampoline frame. So, modified the vdso linker script to discard
unused sections.
Extensions are not implemented.
MFC after: 2 weeks
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
