threading: Support rwlock try_write_lock() on Windows

We explicitly avoided TryAcquireSRWLockExclusive() because of crashes. This
issue was caused by a MinGW-w64 bug (mingw-w64 fix 46f77afc). Using a newer
toolchain works fine.

While try_write_lock() obviously can fail, not supporting it is not really an
option, as some algorithms depend on occasionally successful calls. Certificate
caching in the certificate manager and the cred_set cache rely on successful
try_write_lock()ing.

src/libstrongswan/threading/windows/rwlock.c

@@ -85,8 +85,6 @@ METHOD(rwlock_t, write_lock, void,
 METHOD(rwlock_t, try_write_lock, bool,
 	private_rwlock_t *this)
 {
-	/* TODO: causes random failures and segfaults. Bug? */
-	return FALSE;
 	return TryAcquireSRWLockExclusive(&this->srw);
 }