Call init_cpu_features only if SHARED is defined

In static executable, since init_cpu_features is called early from
__libc_start_main, there is no need to call it again in dl_platform_init.

	[BZ #20072]
	* sysdeps/i386/dl-machine.h (dl_platform_init): Call
	init_cpu_features only if SHARED is defined.
	* sysdeps/x86_64/dl-machine.h (dl_platform_init): Likewise.