selftests/sgx: Separate linker options

Fixes "'linker' input unused [-Wunused-command-line-argument]" errors when
compiling with clang.

Signed-off-by: Jo Van Bulck <jo.vanbulck@cs.kuleuven.be>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/all/20231005153854.25566-5-jo.vanbulck%40cs.kuleuven.be