<math.h>: there's a type-generic __builtin_signbit() now.

No need for libc to manually pick the right type-specific builtin any more.

Change-Id: If615d69ade2917c4b81b33f255c7d045289414dd
diff --git a/libc/include/math.h b/libc/include/math.h
index 3e80083..343ab98 100644
--- a/libc/include/math.h
+++ b/libc/include/math.h
@@ -68,10 +68,7 @@
 
 #define isnormal(x) __builtin_isnormal(x)
 
-#define signbit(x) \
-    ((sizeof(x) == sizeof(float)) ? __builtin_signbitf(x) \
-    : (sizeof(x) == sizeof(double)) ? __builtin_signbit(x) \
-    : __builtin_signbitl(x))
+#define signbit(x) __builtin_signbit(x)
 
 double acos(double __x);
 float acosf(float __x);