[machdep] various fixes on machdeps, __fc_machdep.h generation and tests

- fix various typos and inconsistencies
- add an option in make_machdep.py to validate an existing yaml file,
  and use it to validate manually written machdeps
- add macro with the name of the machdep (used in tests/syntax/assembly_gmp.c)
- add option to Frama-C to print the content of __fc_machdep.h on stdout
  and use that to compile our libc with gcc outside of Frama-C
  (as in tests/libc/runtime.c)