Fix logic in exec_file_locate_attach

Fix logic in exec_file_locate_attach

This commit fixes an error in exec_file_locate_attach where
the main executable could be loaded from outside the sysroot
if a nonempty, non-"target:" sysroot was set but the discovered
executable filename did not exist in that sysroot and did exist
on the main filesystem.

gdb/ChangeLog:

	* exec.c (exec_file_locate_attach): Do not attempt to
	locate main executable locally if not found in sysroot.

gdb/testsuite/ChangeLog:

	* gdb.base/attach-pie-noexec.exp: Do not expect an error
	message on attach.