Fix setting of off-by-default cmake options

Fix setting of off-by-default cmake options

An enclosing CMake project should be able to set the
off-by-default CMake options in SPIRV-Tools by just doing:
  set(SPIRV_SKIP_EXECUTABLES ON)
instead of
  set(SPIRV_SKIP_EXECUTABLES ON CACHE BOOL "" FORCE)

Also, fix the SPIRV_WARN_EVERYTHING so it understands which options
to send to Clang vs. GCC.

Note: With SPIRV_WARN_EVERYTHING enabled, the code doesn't
compil with either Clang or GCC.