[libcxxabi] [test] Avoid test failures with 64/128 bit long doubles on x86

In libcxxabi/src/demangle/ItaniumDemangle.h, we have long double
demangling defined to 20 bytes for i386 and x86_64, based on
explicit arch ifdefs.

For i386 and x86_64 configurations that actually don't use
80 bit long doubles (such as Android), we don't have LDBL_FP80
set within the test, but the demangler is still capable of
demangling 80 bit long doubles, causing test failures 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
    `-----------------------------

Author: mstorsjo

libcxxabi/test/test_demangle.pass.cpp

@@ -30301,7 +30301,7 @@ const char *invalid_cases[] = {
     "_ZNSt16allocator_traitsISaIN4llvm3sys2fs18directory_iteratorEEE9constructIS3_IS3_EEEDTcl12_S_constructfp_fp0_spcl7forwardIT0_Efp1_EEERS4_PT_DpOS7_",
     "3FooILdaaaaaaaaaaAAAAaaEE",
     "3FooILdaaaaaaaaaaaaaaEE",
-#if !LDBL_FP80
+#if !LDBL_FP80 && !defined(__x86_64__) && !defined(__i386__)
     "_ZN5test01hIfEEvRAcvjplstT_Le4001a000000000000000E_c",
 #endif
     // The following test cases were found by libFuzzer+ASAN