mksh: stop loading libc++ and libm.

The dynamic linker unconditionally adds libdl, so this isn't quite down
to just libc, but it's as close as I can get from here.

Unfortunately my timings are in the noise so I can't prove that this
makes any difference in practice, but it can't *hurt*, so....

Test: ldd `which sh`
Change-Id: I9d864120e38d55c85c655ddeaba997cb20e42c6e
diff --git a/Android.bp b/Android.bp
index b7edef2..71a0d67 100644
--- a/Android.bp
+++ b/Android.bp
@@ -115,6 +115,9 @@
         "-DMKSH_DEFAULT_EXECSHELL=\"/system/bin/sh\"",
         "-DMKSH_DEFAULT_TMPDIR=\"/data/local\"",
     ],
+
+    stl: "none",
+    system_shared_libs: ["libc"],
 }
 
 cc_binary {