Add riscv64 support

[mkszuba]

Due to the fact riscv64 is not explicitly mentioned in include/marisa/base.h, MARISA_WORD_SIZE ends up being incorrectly set to 32 on this architecture. Fortunately, this is trivial to address - just add

|| (defined(__riscv) && (__riscv_xlen == 64))

to the end of the #if statement in base.h:31-34.

Confirmed to work on a physical rv64 Linux system.

References:

• output of cpp -dM on an rv64 system
• https://github.com/riscv/riscv-toolchain-conventions