Stop using nowarnings in gdb/testsuite/gdb.multi/

Stop using nowarnings in gdb/testsuite/gdb.multi/

Several of the gdb.multi tests use the "nowarnings" option to suppress
warnings.  The warnings in question all come from missing headers,
like e.g.:

 src/gdb/testsuite/gdb.multi/multi-arch-exec.c:28:3: warning: incompatible implicit declaration of built-in function 'exit' [enabled by default]
    exit (1);
    ^

There's no point in trying to avoid to include standard headers.  In
gdb.base/hangout.c's case, it's even dangerous, as that file calls
printf.  In order to compile a call to a variatic function correctly,
a declaration must be visible.

gdb/testsuite/ChangeLog:
2015-12-10  Pedro Alves  <palves@redhat.com>

	* gdb.multi/base.exp: Don't use nowarnings.
	* gdb.multi/bkpt-multi-exec.exp: Don't use nowarnings.
	* gdb.multi/hangout.c: Include stdio.h.
	* gdb.multi/hello.c: Include stdlib.h.
	* gdb.multi/multi-arch-exec.c: Include stdlib.h.
	* gdb.multi/multi-arch-exec.exp: Don't use nowarnings.
	* gdb.multi/multi-arch.exp: Don't use nowarnings.