[testsuite] fix g++.dg/cpp0x/nullptr21.C to be self-checking

[Janis Johnson]

This patch changes test g++.dg/cpp0x/nullptr21.C to be self-checking
instead of examining printf output via dg-output.  Now it can be run
reliably on bare-metal platforms.  I couldn't see any indication in the
test itself or in the gcc-patches archives to show that the printfs
were significant.

Checked in on mainline, coming soon to 4.7.

Janis
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