Update the FreeBSD system call table to match FreeBSD 12.0.

Update the FreeBSD system call table to match FreeBSD 12.0.

Add a script to generate the FreeBSD XML system call table from the
sys/sys/syscall.h file in the kernel source tree.  For ABI
compatiblity system calls used by older binaries (such as
freebsd11_kevent()), the original system call name is used as an
alias.

Run this script against the current syscall.h file in FreeBSD's head
branch which is expected to be the file used in 12.0 (head is
currently in code freeze as part of the 12.0 release process).

gdb/ChangeLog:

	* syscalls/update-freebsd.sh: New file.
	* syscalls/freebsd.xml: Regenerate.