configure: Merge riscv32 and riscv64 host architectures

The existing code for safe-syscall.inc.S will compile
without change for riscv32 and riscv64.  We may also
drop the meson.build stanza that merges them for tcg/.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>