Add x86 cpuid assembly file.

This patch does the following:

- Adds the generation of crypto/x86cpuid.S in import_openssl.sh.

- Modifies Crypto.mk to ensure that the corresponding functions
  are linked and used at runtime.

Note that mem_clr.c is removed from the x86 build. Its sole purpose
is to provide a generic implementation of OPENSSL_cleanse, which is
provided by the x86cpuid.S source file now.

Change-Id: I7cbf6b12220def11498e591dc64787ef76303c9e