configure: 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)

Since commit ba0e733 ("configure: Merge riscv32 and riscv64
host architectures"), 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)

Now since commit 278c1bc ("target/riscv: Only unify 'riscv32/64'
-> 'riscv' for host cpu in meson") Meson expects the cpu to be in
[riscv32, riscv64]. So when cross-building (for example on our
cross-riscv64-system Gitlab-CI job) we get:

  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

Fix by partially revert commit ba0e733 so when cross-building
the ./configure script passes the proper host architecture to meson.

Fixes: ba0e733 ("configure: Merge riscv32 and riscv64 host architectures")
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: <20230711110619.56588-1-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

configure

@@ -451,7 +451,11 @@ elif check_define __s390__ ; then
     cpu="s390"
   fi
 elif check_define __riscv ; then
-  cpu="riscv"
+  if check_define _LP64 ; then
+    cpu="riscv64"
+  else
+    cpu="riscv32"
+  fi
 elif check_define __arm__ ; then
   cpu="arm"
 elif check_define __aarch64__ ; then