configure: Move the cross_prefix setting after the toolchain one

Makes passing to configure
--toolchain=gcc-asan --cross-prefix=armv7a-hardfloat-linux-gnueabi-
work as intended.

configure

@@ -2494,15 +2494,6 @@ if enabled cross_compile; then
         die "Must specify target arch and OS when cross-compiling"
 fi
 
-ar_default="${cross_prefix}${ar_default}"
-cc_default="${cross_prefix}${cc_default}"
-nm_default="${cross_prefix}${nm_default}"
-pkg_config_default="${cross_prefix}${pkg_config_default}"
-ranlib="${cross_prefix}${ranlib}"
-strip="${cross_prefix}${strip}"
-
-sysinclude_default="${sysroot}/usr/include"
-
 case "$toolchain" in
     clang-asan)
         cc_default="clang"
@@ -2587,6 +2578,15 @@ case "$toolchain" in
     ;;
 esac
 
+ar_default="${cross_prefix}${ar_default}"
+cc_default="${cross_prefix}${cc_default}"
+nm_default="${cross_prefix}${nm_default}"
+pkg_config_default="${cross_prefix}${pkg_config_default}"
+ranlib="${cross_prefix}${ranlib}"
+strip="${cross_prefix}${strip}"
+
+sysinclude_default="${sysroot}/usr/include"
+
 set_default arch cc pkg_config sysinclude target_exec target_os
 enabled cross_compile || host_cc_default=$cc
 set_default host_cc