BOOST_ARCH_WORD_BITS=32 on riscv64

[Kojoley]

This logic is obviously wrong:

predef/include/boost/predef/architecture/riscv.h

Lines 29 to 41 in 614546d

	#if defined(__riscv)
	# undef BOOST_ARCH_RISCV
	# define BOOST_ARCH_RISCV BOOST_VERSION_NUMBER_AVAILABLE
	#endif
	#if BOOST_ARCH_RISCV
	# define BOOST_ARCH_RISCV_AVAILABLE
	#endif
	#if BOOST_ARCH_RISCV
	# undef BOOST_ARCH_WORD_BITS_32
	# define BOOST_ARCH_WORD_BITS_32 BOOST_VERSION_NUMBER_AVAILABLE
	#endif

IIUC __riscv_xlen can be used to detect correct word bitwidth.