[libc] Add a GNU extensions spec containing sincosf from math.h.

[sivachandra]

This is another part I broke off from my sinf/cosf patch.

[abrachet]

LG

libc/spec/gnu_ext.td

+def GnuExtensions : StandardSpec<"GNUExtensions"> {
+  HeaderSpec Math = HeaderSpec<
+      "math.h",
+      [], // Macros
+      [], // Types
+      [], // Enumerations
+      [
+        FunctionSpec<
+            "sincosf",
+            RetValSpec<VoidType>,
+            [ArgSpec<FloatType>, ArgSpec<FloatPtr>, ArgSpec<FloatPtr>]
+        >,
+      ]
+  >;
+
+  let Headers = [
+    Math,
+  ];
+}

libc/spec/spec.td

 def FloatType : NamedType<"float">;
 def DoubleType : NamedType<"double">;
 def LongDoubleType : NamedType<"long double">;
 def CharType : NamedType<"char">;
 
 // Common types
 def VoidPtr : PtrType<VoidType>;
 def SizeTType : NamedType<"size_t">;
+def FloatPtr : PtrType<FloatType>;
 
 // _Noreturn is really not a type, but it is convenient to treat it as a type.
 def NoReturn : NamedType<"_Noreturn void">;
 
 class Macro<string name> {
   string Name = name;
 }