libstdc++: Fix hash<__int128> test for x32 [PR121150]

libstdc++: Fix hash<__int128> test for x32 [PR121150]

I incorrectly assumed that all targets that support __int128 use the
LP64 ABI, so size_t is a 64-bit type. But x32 uses ILP32 and still
supports __int128 (because it's an ILP32 target on 64-bit hardware).

Add casts to the tests so that we get the correct expected values using
size_t type.

libstdc++-v3/ChangeLog:

	PR libstdc++/121150
	* testsuite/20_util/hash/int128.cc: Cast expected values to
	size_t.