Use multilib concept with Clang/ATfE

[rgrr]

These are corrections to the SDK to make my project compile with clang/ATfE 21.1.1. This should be generic.

• using clang multilib/sysroot concept which makes life much easier
• corrected llvm_libc files to make it compile and run with 21.1.1

Final result is:

#       gcc                14.2.1
#              llvm_libc -  FAIL      (mixture of newlib/llvm_libc)
#              picolibc  -  FAIL      (FDEV_SETUP_STREAM is unknown, mixture if newlib/picolibc)
#              newlib    -   OK       (same as -DPICO_CLIB=)

#       clang              19.1.5   21.1.1
#              llvm_libc -  FAIL      OK       (19.1.5: llvm_libc has some incompatibilities)
#              picolibc  -   OK       OK
#              newlib    -   OK       OK       (same as -DPICO_CLIB=)

"Issue" can be found in #2701.

Closes #2701

[lurch]

ping @armandomontanez in case he has any comments 🙂

[rgrr]

Made it draft, because I want to change to multilib