make dg-extra-tool-flags, tool_flags available cleanly

[Janis Johnson]

Procedure dg-test in DejaGnu's dg.exp defines two local variables
dg-extra-tool-flags and tool_flags.  The GCC testsuite support accesses
both of these (using upvar) for several of its local test directives
that determine whether to skip or xfail a test based partly on the flags
that will be used to compile that test.

The GCC testsuite support can eventually be cleaner if DejaGnu makes
those values available to its users, either by making the variables
global (with a better name for tool_flags) or providing functions to
return the values.  Please do so.