selftests/sgx: Use getauxval() to simplify test code

Use the library function getauxval() instead of a custom function to get
the base address of the vDSO.

 [ bp: Massage commit message. ]

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lkml.kernel.org/r/20210314111621.68428-1-tianjia.zhang@linux.alibaba.com