Reorganize the way we handle reserved C macro names.

RESERVED_MACRO_NAMES:
*/RESERVED_MACRO_NAMES:
    Add a macro, _FORTIFY_SOURCE, that gcc defines by default on some recent
    versions of Linux.

    I could have added it in SIX separate RESERVED_MACRO_NAMES files.
    Instead, I moved the content common to all these files to a
    RESERVED_MACRO_NAMES file in the top directory, leaving the
    RESERVED_MACRO_NAMES files of the subdirectories containing only
    macros that *don't* occur in all those subdirectories.

tools/bootcheck:
   Copy the top-level RESERVED_MACRO_NAMES names to the stage 2 directory.

Mmake.common.in:
   Filter out the contents of the top level RESERVED_MACRO_NAMES file,
   as well as the RESERVED_MACRO_NAMES file in the current directory.