osl_get_attribute() should use device strings when compiling for OptiX.

[cmstein]

Description

OSL will call osl_get_attribute() with device strings (ustring hashes) when compiling for the GPU (like osl_printf()).

Tests

No changes to testsuite. Works locally in our renderer.

Checklist:

• I have read the contribution guidelines.
• I have previously submitted a Contributor License Agreement.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

Cliff, do you need a new internal release with this right away?

[cmstein]

Yeah, that'd be helpful. thanks!