Improve USD_BUILD_MODE variable naming

[cpichard]

Arnold usd can build against different flavours of usd: monolithic lib, multiples libs, each can be shared or static. The USD_BUILD_MODE allows three enums: monolithic, static, shared_libs. It can be confusing as a monolithic lib can be compiled in shared or static, and it's not clear if static refers to monolithic or multiple libs.

We propose to rename them: monolithic -> shared_monolithic static -> static_monolithic shared_libs -> shared_libs

[autodesk-oss-arnold-bot[bot]]

Issue synced internally to ARNOLD-13311