LLVM compatibility

[microbit-carlos]

We cannot guarantee continuous support for LLVM toolchain, but work has been carried out to be able to compile CODAL with LLVM instead of GCC.

These are the related patches by @Johnn333:

• codal-core: [Clang compatibility] Preprocessor Directives to ignore builtin clang function codal-core#161
• codal-nrf52:
  • [Clang compatibility] Update nrfx version to 2.1.0 codal-nrf52#48
  • [Clang Compatibility] Patch to define the .heap section as allocatable. codal-nrf52#51
• codal-microbit-nrf5sdk: [Clang compatibility] A SDK_mods compliant Clang compatibility patch. microbit-foundation/codal-microbit-nrf5sdk#16
• microbit-v2-samples: [Clang compatibility] Added Clang CMake definitions. microbit-v2-samples#76

These are some changes needed in codal-microbit-v2 not really mergeable:

• master...Johnn333:codal-microbit-v2-clang:master#diff-5663e8d8a4ee515b0c974ed6dc26b2a737e5d60917f516eeb9ae22dd768b5b66

One of the main barriers to support clang/LLVM would be libc. Arm's LLVM specifically uses picolib:

• https://github.com/ARM-software/LLVM-embedded-toolchain-for-Arm/blob/5db59580875150a9bc438723fb0f79211893802d/docs/migrating.md
• https://github.com/picolibc/picolibc/blob/main/doc/linking.md

[Johnn333]

The patch to codal-nrf52 can be found here: [Clang Compatibility] Patch to define the .heap section as allocatable.