Make the default sysroot be "target:"

Make the default sysroot be "target:"

This commit makes GDB default to a sysroot of "target:".
One testcase needed updating as a result of this change.

gdb/ChangeLog:

	* main.c (captured_main): Set gdb_sysroot to "target:"
	if not otherwise set.

gdb/testsuite/ChangeLog:

	* gdb.base/break-probes.exp: Cope with "target:" sysroot.