Clang/ATfE 21.1.1 fixes

[rgrr]

These are corrections to make my project compile with clang/ATfE 21.1.1. I guess this is more or less generic.

• new layout of include paths of 21.1.1 taken into account
• corrected sysroot to use compilers runtime as specified
• corrected llvm_libc files to make it compile and run with 21.1.1
• make 21.1.1 compile with picolibc (but the result does not run, 19.1.5 does)

newlib is compiling and runnig with 19.1.5 and 21.1.1

Some test results with my project:

#       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 🙂