xinstall: fix invocation of llvm-strip

When executing strip(1), '--' is passed as an argument to explicitly
terminate the getopt(3) loop. The option parsing in llvm-strip doesn't
support this however, so setting XSTRIPBIN=llvm-strip results in an
unsupported argument error. llvm-strip(1) is otherwise
commandline-compatible with FreeBSD's strip(1), so just use the
documented argument format that is common to both.

Special care needs to be taken for filenames beginning with a '-'.

Reviewed by: arichardson, eugen (earlier version, both)
Discussed with: jilles
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D30614