SPI Specific: default to LLVM 12.0.1 (#3187)
SPI Specific: default to LLVM 12.0.1 (#3187)

Co-authored-by: Ole Gulbrandsen <127a8939f3502b11809e346c145e6f3c5c7d47ee@imageworks.com>
OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio