x86/sgx: Make sgx_ipi_cb() available internally

The ETRACK function followed by an IPI to all CPUs within an enclave
is a common pattern with more frequent use in support of SGX2.

Make the (empty) IPI callback function available internally in
preparation for usage by SGX2.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lkml.kernel.org/r/1179ed4a9c3c1c2abf49d51bfcf2c30b493181cc.1652137848.git.reinette.chatre@intel.com