Improve USD_BUILD_MODE variable naming

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