[Bug #20800] Use config target for bin directory as-is

The "target" in `RbConfig::CONFIG` is being changed from config.sub to
align to the system `uname`. Use the value modified by config.sub, and
make the directory same as GNU utilities, such as binutils.

Author: nobu

configure.ac

@@ -149,6 +149,7 @@ dnl checks for alternative programs
 AC_CANONICAL_BUILD
 AC_CANONICAL_HOST
 AC_CANONICAL_TARGET
+AC_SUBST(config_target, $target)
 AS_CASE(["$target_cpu-$target_os"],
     [aarch64-darwin*], [
         target_cpu=arm64

tool/rbinstall.rb

@@ -367,7 +367,7 @@ def CONFIG.[](name, mandatory = false)
 
 bindir = CONFIG["bindir", true]
 if CONFIG["libdirname"] == "archlibdir"
-  archbindir = bindir.sub(%r[/\K(?=[^/]+\z)]) {CONFIG["target"] + "/"}
+  archbindir = bindir.sub(%r[/\K(?=[^/]+\z)]) {CONFIG["config_target"] + "/"}
 end
 libdir = CONFIG[CONFIG.fetch("libdirname", "libdir"), true]
 rubyhdrdir = CONFIG["rubyhdrdir", true]