[ubsan] Add bootstrap-ubsan.mk

[Marek Polacek]

On Tue, Jul 30, 2013 at 04:34:14PM +0200, Marek Polacek wrote:
> This adds the bootstrap-ubsan.mk file so that
> --with-build-config=bootstrap-ubsan be possible.  I doesn't work yet,
> though :(.

One of the reasons is that we use -Werror and e.g. on the following
testcase

static int x;
void
foo (void)
{
  int o = 1;
  x = x >> o;
}

with -O -fsanitize=undefined we generate:
c.c:6:9: warning: ‘x.2’ is used uninitialized in this function [-Wuninitialized]
   x = x >> o;
while with -O -fno-sanitize=undefined there isn't such warning.  When sanitizing,
in .uninit1 we have
  int x.3;
  int x.2;

  <bb 2>: 
  x.3_3 = x.2_1(D) >> 1;
  x = x.3_3;
and when no sanitizing
  int x.1;
  int x.0;

  <bb 2>:
  x.0_2 = x;
  x.1_3 = x.0_2 >> 1;
  x = x.1_3;

The warning comes from warn_uninitialized_vars.  The warning here seems
incorrect, but I'm afraid there isn't much to do about this on the
ubsan side of things.

	Marek