LLVM support on all applicable architectures

[zephyrbot]

Reported by Anas Nashif:

Expand LLVM support to all architectures with LLVM toolchain

(Imported from Jira ZEP-2253)

• ARM support for LLVM/clang Getting LLVM work on ARM #10150

[zephyrbot]

by Mark Linkmeyer:

Moved to 1.10 per review with Chris

[nashif]

depends on Cmake integration.

Some platforms will be able to build, but not link, so we need gcc as well.

[Skjerve]

Is this still on track for 1.12?