meson.build: Merge riscv32 and riscv64 cpu family

In ba0e733362, we merged riscv32 and riscv64 in configure.
However, meson does not treat them the same.  We need to merge
them here as well.

Fixes: ba0e733362
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20211116095042.335224-1-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

meson.build

@@ -59,6 +59,12 @@ supported_cpus = ['ppc', 'ppc64', 's390x', 'riscv', 'x86', 'x86_64',
   'arm', 'aarch64', 'mips', 'mips64', 'sparc', 'sparc64']
 
 cpu = host_machine.cpu_family()
+
+# Unify riscv* to a single family.
+if cpu in ['riscv32', 'riscv64']
+  cpu = 'riscv'
+endif
+
 targetos = host_machine.system()
 
 if cpu in ['x86', 'x86_64']