[RFC] RTEMS Waf Clang support

[Hesham Almatary]

Hi Sebastian,

I have pushed a few commits to my repo to support building RTEMS with
Clang/LLVM for RISC-V. Here are the changes [1]. Since I'm still
learning about Waf and the build system, I'd appreciate your input.
Most importantly, I want to know what's the right way to set "target"
and "xlen" variables.

[1] https://github.com/CTSRD-CHERI/rtems/compare/967b62464bf39602f8b0f2baf57617ad74c3643d...aa73f72b929856efdb41f9f8582263b5395a3276

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Hesham