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

tccelf.c

@@ -1577,7 +1577,7 @@ ST_FUNC void tcc_add_bcheck(TCCState *s1)
         return;
 
     /* XXX: add an object file to do that */
-    ptr = section_ptr_add(bounds_section, sizeof(unsigned long));
+    ptr = section_ptr_add(bounds_section, sizeof(*ptr));
     *ptr = 0;
     add_elf_sym(symtab_section, 0, 0,
                 ELFW(ST_INFO)(STB_GLOBAL, STT_NOTYPE), 0,