[gdb/testsuite] Fix gdb.server/non-existing-program.exp on msys2-ucrt64

[gdb/testsuite] Fix gdb.server/non-existing-program.exp on msys2-ucrt64

On msys2-ucrt64, with test-case gdb.server/non-existing-program.exp I get:
...
(gdb) quit^M
gdb_caching_proc allow_xml_test caused gdb_exit to be called
  gdb_caching_proc allow_xml_test marked as called
  gdb_caching_proc get_mount_point_map marked as called
builtin_spawn gdbserver stdio non-existing-program^M
Error creating process "non-existing-program " (error 2): \
  The system cannot find the file specified.^M^M
Exiting^M^M
FAIL: gdb.server/non-existing-program.exp: gdbserver exits cleanly
...

This happens because this regexp fails to match:
...
    # This is what we get on Windows.
    -re "Error creating process\r\n\r\nExiting\r\n" {
...

Fix this by updating the regexp.

Tested on x86_64-w64-mingw32 (msys2-ucrt64).