provide clean access to elements of dg-do-what

[Janis Johnson]

The GCC testsuite has local support for restricting tests to particular
targets or for marking a test as XFAIL.  In order to do this the local
support uses upvar to access and modify dg-do-what, a local variable in
the DejaGnu procedure dg-test.  This is always done one call away from 
dg-test so it's not as messy as accessing the test name, but it's still
pretty ugly.

Please provide a clean way for a tool to skip or xfail a test, such as
procedures within DejaGnu to modify elements 1 or 2 of dg-do-what, and
to report the current values of those elements.

Janis Johnson