Merge changes from topics "rcar-tools-fix", "toolchain-cleanup" into integration

* changes:
  build: remove the `NM` variable
  build: prefer `gcc-ar` over `ar`
  build: add `--no-warn-rwx-segments` when linking with GCC
  build: always use the C compiler to assemble
  build: always use the C compiler to preprocess
  fix(rcar): fix implicit rule invocations in tools