[gdb/testsuite] Fix tcl error in cached_file

[gdb/testsuite] Fix tcl error in cached_file

When trying to run tests using target board cc-with-dwz after a clean build, I
run into:
...
ERROR: tcl error sourcing board description file for target cc-with-tweaks.exp.
couldn't open "build/gdb/testsuite/cache/gdb.sh.17028": \
  no such file or directory
couldn't open "build/gdb/testsuite/cache/gdb.sh.17028": \
  no such file or directory
    while executing
"open $tmp_filename w"
    (procedure "cached_file" line 9)
    invoked from within
"cached_file gdb.sh "$GDB $INTERNAL_GDBFLAGS $GDBFLAGS \"\$@\"" 1"
...

The problem is that cached_file is trying to create a file
build/gdb/testsuite/cache/gdb.sh.17028 in a non-existing directory.

Fix this by creating the cache dir in cached_file.

Tested on x86_64-linux, with target board cc-with-tweaks, and native.

gdb/testsuite/ChangeLog:

2020-03-09  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (cached_file): Create cache dir.