Run argv0-symlink.exp only on native target and local host.

Run argv0-symlink.exp only on native target and local host.

This testcase was originally for PR gdb/15415, a problem with the
"run" command expanding symlinks in the name of the program being run.
It does not correctly distinguish between files on build, host, and
target, and it is not clear if it would be testing anything useful in
configurations where "run" is not being used.

2019-08-29  Sandra Loosemore  <sandra@codesourcery.com>

	* gdb.base/argv0-symlink.exp: Run only on native target
	and local host.