Implement decoding of pidfd_send_signal syscall

Implement decoding of pidfd_send_signal syscall

... introduced by Linux kernel commit v5.1-rc1~6^2~1.

* signal.c (SYS_FUNC(pidfd_send_signal)): New function.
* linux/32/syscallent.h [424]: Wire up pidfd_send_signal.
* linux/64/syscallent.h: Likewise.
* linux/arm/syscallent.h: Likewise.
* linux/hppa/syscallent.h: Likewise.
* linux/i386/syscallent.h: Likewise.
* linux/m68k/syscallent.h: Likewise.
* linux/microblaze/syscallent.h: Likewise.
* linux/powerpc/syscallent.h: Likewise.
* linux/powerpc64/syscallent.h: Likewise.
* linux/s390/syscallent.h: Likewise.
* linux/s390x/syscallent.h: Likewise.
* linux/sh/syscallent.h: Likewise.
* linux/sh64/syscallent.h: Likewise.
* linux/sparc/syscallent.h: Likewise.
* linux/sparc64/syscallent.h: Likewise.
* linux/x32/syscallent.h: Likewise.
* linux/x86_64/syscallent.h: Likewise.
* linux/xtensa/syscallent.h: Likewise.
* linux/alpha/syscallent.h [534]: Likewise.
* linux/ia64/syscallent.h [1024 + 424]: Likewise.
* linux/mips/syscallent-n32.h [6424]: Likewise.
* linux/mips/syscallent-n64.h [5424]: Likewise.
* linux/mips/syscallent-o32.h [4424]: Likewise.
* NEWS: Mention this change.
* tests/pidfd_send_signal.c: New file.
* tests/gen_tests.in (pidfd_send_signal): New entry.
* tests/pure_executables.list: Add pidfd_send_signal.
* tests/.gitignore: Likewise.