Linux: Consolidate auxiliary vector parsing

And optimize it slightly.

The large switch statement in _dl_sysdep_start can be replaced with
a large array.  This reduces source code and binary size.  On
i686-linux-gnu:

Before:

   text	   data	    bss	    dec	    hex	filename
   7791	     12	      0	   7803	   1e7b	elf/dl-sysdep.os

After:

   text	   data	    bss	    dec	    hex	filename
   7135	     12	      0	   7147	   1beb	elf/dl-sysdep.os

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

sysdeps/unix/sysv/linux/powerpc/dl-support.c

@@ -0,0 +1,4 @@
+#include <elf/dl-support.c>
+
+/* Populated from the auxiliary vector.  */
+int _dl_cache_line_size;