Attached is another revision of this enhancement, this one
incorporating Richard's request for a more efficient encoding
of the attributes to enable faster parsing.  There is just one
attribute called access, with the rest being arguments.  This
is transformed to access (mode-string) where the mode-string
is a STRING_CST describing the access mode (read/write/both)
and the two positional arguments for the function type.

I have also removed the attributes from the built-in functions
since they're not used for anything yet, and added more argument
validation.

To test this a little more extensively, I annotated a few Glibc
<unistd.h> functions with the new attribute and rebuilt it and
its test suite.  That exposed a problem with function pointers
not being handled correctly so I fixed that by letting
the access attribute apply to function pointers (and function
pointer types in general).

When this is finalized, if there's time I'm still hoping to get
back to the parts of the patch that make use of the attribute
for -Wunused and -Wuninitialized that I removed on Jeff's request
for smaller, independent changes.

In GCC 11 I'd like to look into tying this attribute in with
_FORTIFY_SOURCE.

Martin