Fix floating point unary minus and plus

negate(x) is subtract(-0,x), not subtract(+0,x), which makes
a difference with signed zeros.  Also +x was expressed as x+0,
in order for the integer promotions to happen, but also mangles signed
zeros, so just don't do that with floating types.

tests/tcctest.c

@@ -1699,6 +1699,29 @@ void prefix ## call(void)\
     printf("strto%s: %f\n", #prefix, (double)strto ## prefix("1.2", NULL));\
 }\
 \
+void prefix ## signed_zeros(void) \
+{\
+  type x = 0.0, y = -0.0, n, p;\
+  if (x == y)\
+    printf ("Test 1.0 / x != 1.0 / y  returns %d (should be 1).\n",\
+            1.0 / x != 1.0 / y);\
+  else\
+    printf ("x != y; this is wrong!\n");\
+\
+  n = -x;\
+  if (x == n)\
+    printf ("Test 1.0 / x != 1.0 / -x returns %d (should be 1).\n",\
+            1.0 / x != 1.0 / n);\
+  else\
+    printf ("x != -x; this is wrong!\n");\
+\
+  p = +y;\
+  if (x == p)\
+    printf ("Test 1.0 / x != 1.0 / +y returns %d (should be 1).\n",\
+            1.0 / x != 1.0 / p);\
+  else\
+    printf ("x != +y; this is wrong!\n");\
+}\
 void prefix ## test(void)\
 {\
     printf("testing '%s'\n", #typename);\
@@ -1708,6 +1731,7 @@ void prefix ## test(void)\
     prefix ## fcast(234.6);\
     prefix ## fcast(-2334.6);\
     prefix ## call();\
+    prefix ## signed_zeros();\
 }
 
 FTEST(f, float, float, "%f")