[DISCO_F746NG] Use Cortex-M7 instead of Cortex-M7F

Using Cortex-M7F, singletest.py does not work (build error).

workspace_tools/targets.py

@@ -777,7 +777,7 @@ class DISCO_L053C8(Target):
 class DISCO_F746NG(Target):
     def __init__(self):
         Target.__init__(self)
-        self.core = "Cortex-M7F"
+        self.core = "Cortex-M7"
         self.extra_labels = ['STM', 'STM32F7', 'STM32F746', 'STM32F746NG']
         self.supported_toolchains = ["ARM", "uARM", "IAR"]
         self.default_toolchain = "uARM"