When running the DejaGNU testsuite on a toolchain built for native
Windows, the path /dev/null can't be used to open a stream to void.
On native Windows, the resource is instead named "nul".

In 17_intro/tag_type_explicit_ctor.cc, the following statement would
fail to match when the DejaGNU testsuite is running in cygwin with a
native toolchain.
// dg-error 53 "explicit" "" { target hosted }

The "target hosted"-check is using cpp to verify if _GLIBCXX_HOSTED is
defined and discards the output by simply redirecting it to /dev/null.
In v3_target_compile, it's overridden to "nul" for MinGW targets, but
the same rule applies when host is cygwin, so replace the condition
with a check for Windows.

The error in the log would look like this for the "target hosted" check:
cc1plus.exe: fatal error: opening output file /dev/null: No such file or directory

The tag_type_explicit_ctor.cc test fails with this on Windows:
.../tag_type_explicit_ctor.cc:53: error: converting to 'std::defer_lock_t' from initializer list would use explicit constructor 'constexpr std::defer_lock_t::defer_lock_t()'
.../tag_type_explicit_ctor.cc:54: error: converting to 'std::try_to_lock_t' from initializer list would use explicit constructor 'constexpr std::try_to_lock_t::try_to_lock_t()'
.../tag_type_explicit_ctor.cc:55: error: converting to 'std::try_to_lock_t' from initializer list would use explicit constructor 'constexpr std::try_to_lock_t::try_to_lock_t()'
.../tag_type_explicit_ctor.cc:67: error: converting to 'std::defer_lock_t' from initializer list would use explicit constructor 'constexpr std::defer_lock_t::defer_lock_t()'
.../tag_type_explicit_ctor.cc:68: error: converting to 'std::try_to_lock_t' from initializer list would use explicit constructor 'constexpr std::try_to_lock_t::try_to_lock_t()'
.../tag_type_explicit_ctor.cc:69: error: converting to 'std::adopt_lock_t' from initializer list would use explicit constructor 'constexpr std::adopt_lock_t::adopt_lock_t()'

Patch has been verified on Windows and Linux.

libstdc++-v3:

	* testsuite/lib/libstdc++.exp: Use "nul" for Windows, "/dev/null"
	for other environments.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>