C89isms in the test suite

[Florian Weimer <fweimer@redhat.com>]

What should we do about these when they are not relevant to what's being
tested?  For example, gcc/testsuite/gcc.c-torture/execute/ieee/mzero6.c
has this:

  int main ()
  {
    if (__builtin_copysign (1.0, func (0.0 / -5.0, 10)) != -1.0)
      abort ();
    exit (0);
  }

but no include files, so abort and exit are implicitly declared.

Should we inject a header with -include with the most common
declarations (which includes at least abort and exit)?  Or add the
missing #include directives?  But the latter might not work for
freestanding targets.

Implicit ints and function declarations without prototypes are also
common (not just for main).

Other tests look like they might be intended to be built in C89 mode,
e.g.  gcc/testsuite/gcc.c-torture/compile/386.c, although it's not
immediately obvious to me what they test.

Thanks,
Florian