Add RISC-V support

[tingsi5]

We have an Embree RISC-V port based on sse2rvv [1] for a project that uses SCons as the build system. If we would like to contribute it back to the Embree project, what's the proper way to add it to the build system?

In CMakeList.txt, it checks CMAKE_SYSTEM_PROCESSOR to set EMBREE_ARM for aarch64 [2], which seems to expect building Embree on the same architecture. Is this a convention that needs to follow? Or it's CMAKE_TOOLCHAIN_FILE to use for cross-compiling, or are there any other options you'd suggest? Thank you.

[1] https://github.com/pattonkan/sse2rvv
[2]

embree/CMakeLists.txt

Lines 238 to 240 in 0c236df

	ELSEIF(CMAKE_SYSTEM_PROCESSOR STREQUAL "aarch64" OR CMAKE_SYSTEM_PROCESSOR STREQUAL "ARM64")
	MESSAGE(STATUS "Building for AArch64")
	SET(EMBREE_ARM ON)

[tingsi5]

Close as #503.