gcc: remove --posixly, add -9

The GCC option `--posixly` was introduced to enable the compilation
of POSIX code in Jehanne, assuming that building native C code
would have been the default.

However since

- GCC is itself a POSIX program and most programmers that use it
  assume a POSIX environment
- GCC is used as a cross compiler to build Jehanne from POSIX systems
- C++ will never be native in Jehanne (too much complexity for a
  system that want to be simple) and thus will always require a
  POSIX environment to build POSIX C++ code

I decided to invert the logic.

Thus now by default GCC, by default, will build POSIX programs,
loading required libraries and headers from /posix subdirectories.

The new option `-9` will remove such POSIX stuff when compiling
native C programs.

cross/patch/gcc/gcc/config/jehanne.opt

@@ -18,8 +18,8 @@
 
 ; Please try to keep this file in ASCII collating order.
 
--posixly
-C ObjC C++ ObjC++ Driver RejectNegative
-Lookup for POSIX includes and libraries in /posix
+9
+C Driver RejectNegative
+Do not lookup for POSIX includes and libraries in /posix
 
 ; This comment is to ensure we retain the blank line above.