Fix Windows-target testing in gdb_file_cmd

GCC for Windows target produces executables called foo.exe when given
"-o foo".  (More specifically, it's done that for native compilers for
a long time, and for cross compilers to Windows target since GCC
commit 5bc86b599054f494ec0a45e49b82749320eaa9c4, in GCC 8 and later.)
This causes problems for many GDB tests expecting a program to have
the exact file name passed to -o.

Fix this by checking for the case where only the .exe exists in
gdb_file_cmd and adjusting the name passed to the file command
accordingly.  There may well be other places with this issue in the
GDB testsuite, but this fix allows many tests to succeed that
previously fell over.

2020-11-12  Joseph Myers  <joseph@codesourcery.com>

	* lib/gdb.exp (gdb_file_cmd): Check for case where $arg.exe exists
	but $arg does not.