lib and doc: Use "libSystem" as identifier for that libc in platforms

doc/cross-compilation.xml

···
         <listitem>
           <para>
             This is a string identifying the standard C library used.
-            Valid identifiers include "glibc" for GNU libc, "libsystem" for Darwin's Libsystem, and "uclibc" for µClibc.
+            Valid identifiers include "glibc" for GNU libc, "libSystem" for Darwin's Libsystem, and "uclibc" for µClibc.
             It should probably be refactored to use the module system, like <varname>parse</varname>.
           </para>
         </listitem>

lib/systems/examples.nix

···
   iphone64 = {
     config = "aarch64-apple-darwin14";
     arch = "arm64";
-    libc = "libsystem";
+    libc = "libSystem";
     platform = {};
   };
 
   iphone32 = {
     config = "arm-apple-darwin10";
     arch = "armv7-a";
-    libc = "libsystem";
+    libc = "libSystem";
     platform = {};
   };