[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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-03-09  Tom de Vries  <tdevries@suse.de>
+
+	* lib/gdb.exp (cached_file): Create cache dir.
+
 2020-03-07  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.cp/ref-params.exp: Compare build_executable result with -1.

gdb/testsuite/lib/gdb.exp

@@ -4958,6 +4958,9 @@ proc cached_file { filename txt {executable 0}} {
 	return $filename
     }
 
+    set dir [file dirname $filename]
+    file mkdir $dir
+
     set tmp_filename $filename.[pid]
     set fd [open $tmp_filename w]
     puts $fd $txt