Pass INTERNAL_GDBFLAGS when executing GDB

Pass INTERNAL_GDBFLAGS when executing GDB

gdb.debuginfod/fetch_src_and_symbols.exp attempts to ascertain
whether GDB was built with debuginfod support by executing
"$GDB --configuration".

That seems harmless enough. However, if GDB is not already installed
on the host, the command will fail:

$ ./gdb --config
Exception caught while booting Guile.
Error in function "open-file":
No such file or directory: "/usr/share/gdb/guile/gdb/boot.scm"
./gdb: warning: Could not complete Guile gdb module initialization from:
/usr/share/gdb/guile/gdb/boot.scm.
Limited Guile support is available.
Suggest passing --data-directory=/path/to/gdb/data-directory.
Python Exception <class 'ModuleNotFoundError'> No module named 'gdb':
./gdb: warning:
Could not load the Python gdb module from `/usr/share/gdb/python'.
Limited Python support is available from the _gdb module.
Suggest passing --data-directory=/path/to/gdb/data-directory.
This GDB was configured as follows:
   configure --host=x86_64-redhat-linux-gnu --target=x86_64-redhat-linux-gnu
      [abbreviated output]

The problem here is, of course, that while running in the test suite,
we must pass INTERNAL_GDBFLAGS in order to pick up the --data-directory
option.

gdb/testsuite/ChangeLog
2020-06-17  Keith Seitz  <keiths@redhat.com>

	* gdb.deuginfod/fetch_src_and_symbols.exp: Pass INTERNAL_GDBFLAGS
	when executing "gdb --configuration".