fix for the bcheck.o (bug #14958)

fix for the bcheck.o (bug #14958)

    - care about __attribute__ redefinition in the system headers
    - an invalid pointer must be returned when (addr >= e->size),
      and not (addr > e->size)

    A test program:
    #include <stdio.h>
    #include <stdlib.h>
    int main ()
    {
	int v[10];
	fprintf(stderr, "&v[0]  = %p\n", &v[0]);
	fprintf(stderr, "&v[10] = %p\n", &v[10]);
	exit(1);
	return 0;
    }
    // tcc -b test.c

    The output before a patch:
    &v[0]  = 0xbf929d8c
    &v[10] = 0xbf929db4

    The output after a patch:
    &v[0]  = 0xbff6e33c
    &v[10] = 0xfffffffe