Use errno value of first openp failure

Use errno value of first openp failure

Fixes this testsuite fail on Windows:
FAIL: gdb.base/bad-file.exp: directory

If both tries to open the file fail (without and with ".exe"), use the
errno value of the first try.

gdb/ChangeLog:

2020-05-27  Hannes Domani  <ssbssa@yahoo.de>

	* exec.c (exec_file_attach): Use errno value of first openp failure.