configure: Fix ppc container_cross_cc substitution

When moving this code out of probe_target_compiler(), we failed to adjust
the variable in which the target is located, resulting in e.g.
powerpc64-linux-user-linux-gnu-gcc-10

Fixes: cd362de ("tests/tcg: merge configure.sh back into main configure script")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220728183901.1290113-1-richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -2028,7 +2028,7 @@ probe_target_compiler() {
         ;;
       ppc64|ppc64le)
         container_image=debian-powerpc-test-cross
-        container_cross_prefix=powerpc${1#ppc}-linux-gnu-
+        container_cross_prefix=powerpc${target_arch#ppc}-linux-gnu-
         container_cross_cc=${container_cross_prefix}gcc-10
         ;;
       riscv64)