meson: Fix cross-building for RISCV host

While when building on native Linux the host architecture
is reported as "riscv32" or "riscv64":

  Host machine cpu family: riscv64
  Host machine cpu: riscv64
  Found pkg-config: /usr/bin/pkg-config (0.29.2)

When cross-compiling it is detected as "riscv". Meson handles
the cross-detection but displays a warning:

  WARNING: Unknown CPU family riscv, please report this at https://github.com/mesonbuild/meson/issues/new
  Host machine cpu family: riscv
  Host machine cpu: riscv
  Target machine cpu family: riscv
  Target machine cpu: riscv
  Found pkg-config: /usr/bin/riscv64-linux-gnu-pkg-config (1.8.1)

Commit 278c1bc was tested on native host but not under
cross environment, and now we get on our cross-riscv64-system
Gitlab-CI job:

  WARNING: Unknown CPU family riscv, please report this at https://github.com/mesonbuild/meson/issues/new
  Host machine cpu family: riscv
  Host machine cpu: riscv
  Target machine cpu family: riscv
  Target machine cpu: riscv
  ../meson.build:684:6: ERROR: Problem encountered: Unsupported CPU riscv, try --enable-tcg-interpreter

As a kludge, re-introduce "riscv" in the supported_cpus[] array.

Fixes: 278c1bc ("target/riscv: Only unify 'riscv32/64' -> 'riscv' for host cpu in meson")
Reported-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230711094147.54985-1-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

meson.build

@@ -55,7 +55,7 @@ qapi_trace_events = []
 
 bsd_oses = ['gnu/kfreebsd', 'freebsd', 'netbsd', 'openbsd', 'dragonfly', 'darwin']
 supported_oses = ['windows', 'freebsd', 'netbsd', 'openbsd', 'darwin', 'sunos', 'linux']
-supported_cpus = ['ppc', 'ppc64', 's390x', 'riscv32', 'riscv64', 'x86', 'x86_64',
+supported_cpus = ['ppc', 'ppc64', 's390x', 'riscv', 'riscv32', 'riscv64', 'x86', 'x86_64',
   'arm', 'aarch64', 'loongarch64', 'mips', 'mips64', 'sparc64']
 
 cpu = host_machine.cpu_family()