fixincludes: simplify handling for access() failure [PR21283, PR80047]

fixincludes: simplify handling for access() failure [PR21283, PR80047]

POSIX says:

    On some implementations, if buf is a null pointer, getcwd() may obtain
    size bytes of memory using malloc(). In this case, the pointer returned
    by getcwd() may be used as the argument in a subsequent call to free().
    Invoking getcwd() with buf as a null pointer is not recommended in
    conforming applications.

This produces an error building GCC with --enable-werror-always:

    ../../../fixincludes/fixincl.c: In function ‘process’:
    ../../../fixincludes/fixincl.c:1356:7: error: argument 1 is null but
    the corresponding size argument 2 value is 4096 [-Werror=nonnull]

It's suggested by POSIX to call getcwd() with progressively larger
buffers until it does not give an [ERANGE] error. However, it's highly
unlikely that this error-handling route is ever used.

So we can simplify it instead of writting too much code.  We give up to
use getcwd(), because `make` will output a `Leaving directory ...` message
containing the path to cwd when we call abort().

fixincludes/ChangeLog:

	PR other/21823
	PR bootstrap/80047
	* fixincl.c (process): Simplify the handling for highly
	  unlikely access() failure, to avoid using non-standard
	  extensions.