[libcxxabi] Use __LDBL_MANT_DIG__ for configuring demangling of long doubles

This avoids needing to hardcode the mapping between architectures
and their sizes of long doubles.

This fixes a case in test_demangle.pass.cpp, that previously failed
like this:

    .---command stdout------------
    | Testing 29859 symbols.
    | _ZN5test01hIfEEvRAcvjplstT_Le4001a000000000000000E_c should be invalid but is not
    | Got: 0, void test0::h<float>(char (&) [(unsigned int)(sizeof (float) + 0x0.07ff98f7ep-1022L)])
    `-----------------------------
    .---command stderr------------
    | Assertion failed: !passed && "demangle did not fail", file libcxxabi/test/test_demangle.pass.cpp, line 30338
    `-----------------------------

This testcase is defined within

    // Is long double fp80?  (Only x87 extended double has 64-bit mantissa)
    #define LDBL_FP80 (__LDBL_MANT_DIG__ == 64)
    ...
    #if !LDBL_FP80
    ...
    #endif

The case failed, by unexpectedly passing, when the demangler was
capable of demangling an 80 bit long double, while the compiler
actually is configured for a different size for that type.

Author: mstorsjo

libcxxabi/src/demangle/ItaniumDemangle.h

@@ -5739,13 +5739,11 @@ struct FloatData<double>
 template <>
 struct FloatData<long double>
 {
-#if defined(__mips__) && defined(__mips_n64) || defined(__aarch64__) || \
-    defined(__wasm__) || defined(__riscv) || defined(__loongarch__) || \
-    defined(__ve__)
+#if __LDBL_MANT_DIG__ == 113
     static const size_t mangled_size = 32;
-#elif defined(__arm__) || defined(__mips__) || defined(__hexagon__)
+#elif __LDBL_MANT_DIG__ == 53
     static const size_t mangled_size = 16;
-#else
+#else // __LDBL_MANT_DIG__ == 64
     static const size_t mangled_size = 20;  // May need to be adjusted to 16 or 24 on other platforms
 #endif
     // `-0x1.ffffffffffffffffffffffffffffp+16383` + 'L' + '\0' == 42 bytes.