Default compute dimensions (runtime)

[Nathan Sidwell]

Jakub,
this is the runtime side of default compute dimension support.

1) extend the -fopenacc-dim=X:Y:Z syntax to allow '-' indicating a runtime 
choice.  (0 also indicates that, but I thought best to have an explicit syntax 
as well).

2) New plugin helper 'GOMP_PLUGIN_acc_default_dims' that parses a 
GOMP_OPENACC_DIM environment variable.  The syntax here is the same as that for 
the -fopenacc-dim option -- except '-' isn't permitted.  I have future-proofed 
the interface by including a plugin tag parameter.  This  will permit 
device_type support.

3) the plugin itself lazily calls GOMP_PLUGIN_acc_default_dims when it sees an 
unspecified dimension.  Validates the default dimensions and then plugs them 
into the launch parameters.

The testcase reuses the compile-time testcase by breaking its core to a header 
file and explicitly setting the environment variable before first launch.  The 
original testcase also explitily sets  the environment variable, to make sure 
it's not being considered.

There doesn't seem to be a mechanism warning messages -- only debug ones or 
fatal errors.  I'm not sure what the best approach to handling errors in the env 
var parsing, and ducked to silently ignore problems (and the plugin will then 
provide fallback values).

ok?

nathan
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