softfloat: Prepend QEMU-style header with derivation notice

The SoftFloat license requires "prominent notice that the work
is derivative". Having added features like improved 16-bit support
for arm already, add such a notice to the sources.

softfloat-native.[ch] are not under the SoftFloat license
and thus are not changed.

Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

fpu/softfloat-macros.h

+/*
+ * QEMU float support macros
+ *
+ * Derived from SoftFloat.
+ */
 
 /*============================================================================
 

fpu/softfloat-specialize.h

+/*
+ * QEMU float support
+ *
+ * Derived from SoftFloat.
+ */
 
 /*============================================================================
 

fpu/softfloat.c

+/*
+ * QEMU float support
+ *
+ * Derived from SoftFloat.
+ */
 
 /*============================================================================
 

fpu/softfloat.h

+/*
+ * QEMU float support
+ *
+ * Derived from SoftFloat.
+ */
+
 /*============================================================================
 
 This C header file is part of the SoftFloat IEC/IEEE Floating-point Arithmetic