[PATCH][GCC] Fix native Windows x86 bootstrap failure with self test

[Tamar Christina]

Hi all,

The GCC self-test added in r237144 breaks the
native Windows x86 builds (e.g. mingw-w64).
This fixes (PR78196) by explicitly adding /dev/null as
the output file to the GCC self test.

The test essentially does `-xc -S -c /dev/null -fself-test`

`/dev/null` is then converted into the Windows null device `nul`
by the MSYS shell, which is correct. But then the driver adds a
filename to the name, trying to write the output to `nul.s`.
`nul` is a reserved filename on Windows. As such it's invalid
to create this file and the call always fails using CreateFile.

Checked with x86_64-w64-mingw32 and build gets passed self tests
but dies at unrelated libstdc++ and libvtv errors.

Ok for trunk?

Thanks,
Tamar

gcc/

2016-11-03  Tamar Christina  <tamar.christina@arm.com>

	PR driver/78196
	* Makefile.in (SELFTEST_FLAGS): Added -o /dev/null.
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