target-arm/helper.c: Don't assume softfloat int32 is 32 bits only

In the helper routines for VCVT float-to-int conversions, add
an explicit cast rather than relying on the softfloat int32
type being exactly 32 bits wide (which it is not guaranteed to be).
Without this, if the softfloat type was 64 bits wide we would
get zero-extension of the 32 bit value from the ARM register
rather than sign-extension, since TCG i32 values are passed as
uint32_t.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target-arm/helper.c

@@ -2785,7 +2785,7 @@ DO_VFP_cmp(d, float64)
     float##fsz HELPER(name)(uint32_t x, void *fpstp) \
 { \
     float_status *fpst = fpstp; \
-    return sign##int32_to_##float##fsz(x, fpst); \
+    return sign##int32_to_##float##fsz((sign##int32_t)x, fpst); \
 }
 
 #define CONV_FTOI(name, fsz, sign, round) \