Refactor: abstract SDK interface for `libsgxstep` about sgx-step

[jovanbulck]

some relevant notes on moving fwd here; currently we rely on the following patches (which can probably be further reduced as outlined):

• sgx_get/set_aep: this is the only crucial part: we must be able to set our custom aep_trampoline.S stub on EENTER-ing the enclave -- current patch does not work with the VDSO eenter provided by newer Linux kernels..
• sgx_get_tcs: to query TCS address that is saved on first EENTER -- we can probably get rid of this and by saving the TCS in rbx in SGX-Step's aep_trampoline.S

TODO: look into how to best support __vdso_sgx_enter_enclave; maybe we can trick the vdso_detector to take our customized vdso_sgx_enter_enclave function so we wouldn't need any patches in the SGX SDK runtime ^^

Note: Linux VDSO system sets the AEP here

[jovanbulck]

idea to look into: maybe we can simply replace all enclu instructions in VDSO and shared libraries with ud2 and trap these with a users-space signal handler to automatically redirect to our stub without needing to recompile or patch any untrusted runtime..