Add char8_t name mangling (Du) and RTTI support.

Fixes #45.

Author: tahonermann

abi.html

@@ -2133,7 +2133,7 @@ <h4><a href="#rtti-emission"> 2.9.2 Place of Emission </a></h4>
 for every X in: void, std::nullptr_t,
 bool, wchar_t, char, unsigned char, signed char,
 short, unsigned short, int, unsigned int, long, unsigned long, long long,
-unsigned long long, float, double, long double, char16_t, char32_t,
+unsigned long long, float, double, long double, char8_t, char16_t, char32_t,
 and the IEEE 754r decimal and half-precision floating point types.
 Each of the type_info objects for X shall have type
 <code>abi::__fundamental_type_info</code> (or a type derived therefrom),
@@ -4737,6 +4737,7 @@ <h5><a href="#mangling-builtin">5.1.5.2 Builtin types</a></h5>
                  ::= DF &lt;<a href="#mangle.number">number</a>&gt; _ # ISO/IEC TS 18661 binary floating point type _FloatN (N bits)
                  ::= Di # char32_t
                  ::= Ds # char16_t
+                 ::= Du # char8_t
                  ::= Da # auto
                  ::= Dc # decltype(auto)
                  ::= Dn # std::nullptr_t (i.e., decltype(nullptr))