cmake: detect sysroot for cross-compile toolchains

Fixes: #49587

Try to detect sysroot for cross-compile toolchain if not specified by
the user with `-DSYSROOT_DIR=<path>`.

First, the C compiler is asked if it knows its sysroot, some C compilers
are able to return the sysroot, as an example:
> $ arm-none-eabi-gcc -print-sysroot
> <path>/bin/../arm-none-eabi
> $

but majority of gcc-based C compilers seems to return empty string:
> $ arm-zephyr-eabi-gcc --print-sysroot
> $

in such cases, the cross-compiler target CMake file will try to discover
the sysroot by searching for libc.a, starting one level up from the
compiler location.

If no sysroot candidate is found, a warning will be printed to the user.
If a single sysroot candidate is found, this candidate will be selected.
If multiply sysroot candidates are found, a warning is printed, and the
first candiate in the list is selected. User may select another
candidate with `-DSYSROOT_DIR=<use-this>`.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

cmake/toolchain/cross-compile/target.cmake

@@ -1,3 +1,64 @@
 # SPDX-License-Identifier: Apache-2.0
 
-# This file intentionally left blank.
+if(NOT SYSROOT_DIR)
+  # When using cross-compile, the generic toolchain will be identical to the
+  # target toolchain, hence let's ask the compiler if it knows its own sysroot.
+  # Some gcc based compilers do have this information built-in.
+  execute_process(COMMAND ${CMAKE_C_COMPILER} --print-sysroot
+                  RESULT_VARIABLE return_val
+                  OUTPUT_VARIABLE reported_sysroot
+                  OUTPUT_STRIP_TRAILING_WHITESPACE
+  )
+  if(NOT return_val AND NOT "${reported_sysroot}" STREQUAL "")
+    set(SYSROOT_DIR "${reported_sysroot}" CACHE INTERNAL "Sysroot reported by ${CMAKE_C_COMPILER}")
+  else()
+    # Let's try to lookup a sysroot.
+    # Compiler is expected to be located in `bin/` so we go one folder up for search.
+    cmake_path(GET CMAKE_C_COMPILER PARENT_PATH search_path)
+    cmake_path(SET search_path NORMALIZE "${search_path}/..")
+    file(GLOB_RECURSE libc_dirs RELATIVE ${search_path} ${search_path}/**/libc.a )
+
+    # Did we find any candidates ?
+    list(LENGTH libc_dirs libc_dirs_length)
+    if(${libc_dirs_length} GREATER 0)
+      list(TRANSFORM libc_dirs REPLACE "libc.a$" "")
+      list(SORT libc_dirs COMPARE NATURAL)
+      list(GET libc_dirs 0 sysroot)
+      set(sysroot_candidates ${sysroot})
+
+      foreach(dir ${libc_dirs})
+        string(FIND "${dir}" "${sysroot}" index)
+        if(${index} EQUAL -1)
+          set(sysroot ${dir})
+          list(APPEND sysroot_candidates ${sysroot})
+        endif()
+      endforeach()
+
+      # We detect the lib folders, the actual sysroot is up one level.
+      list(TRANSFORM sysroot_candidates REPLACE "/lib[/]?$" "")
+
+      list(LENGTH sysroot_candidates sysroot_candidates_length)
+      if(sysroot_candidates_length GREATER 1)
+        list(TRANSFORM sysroot_candidates PREPEND "${search_path}")
+        string(REPLACE ";" "\n" sysroot_candidates_str "${sysroot_candidates}")
+
+        message(WARNING "Multiple sysroot candidates found: ${sysroot_candidates_str}\n"
+                        "Use `-DSYSROOT_DIR=<sysroot-dir>` to select sysroot.\n"
+        )
+      endif()
+
+      # Use first candidate as sysroot.
+      list(GET sysroot_candidates 0 chosen_sysroot)
+      cmake_path(SET SYSROOT_DIR NORMALIZE "${search_path}/${chosen_sysroot}")
+    endif()
+
+    if(SYSROOT_DIR)
+      set(SYSROOT_DIR "${SYSROOT_DIR}" CACHE INTERNAL "Sysroot discovered")
+      message(STATUS "Found sysroot: ${SYSROOT_DIR}")
+    else()
+      message(WARNING "No sysroot found, build may not work.\n"
+                      "Use `-DSYSROOT_DIR=<sysroot-dir>` to specify sysroot to use.\n"
+      )
+    endif()
+  endif()
+endif()