fixup! [libc][math][c23] Add fabsf16 C23 math function

Fix usage of internal float16 alias in generated headers.

Author: overmighty

libc/spec/spec.td

@@ -53,7 +53,7 @@ def UnsignedCharType : NamedType<"unsigned char">;
 def UnsignedShortType : NamedType<"unsigned short">;
 def BoolType : NamedType<"bool">;
 
-def Float16Type : NamedType<"float16">;
+def Float16Type : NamedType<"_Float16">;
 def Float128Type : NamedType<"float128">;
 
 // Common types