libsgx_enclave_common: enclave_load_data doesn't configure page tables correctly

`enclave_load_data` will call `mprotect` to match the PT permissions to the EPCM permissions. Unfortunately, when calling `mprotect` with `PROT_NONE`, Linux will completely remove the page table mapping instead of mapping it without permissions. This is a problem for TCS pages which don't require any EPCM permissions.

Suggested resolution: protect such pages with `PROT_READ` instead.

Alternatively, since the PT permissions don't really matter, and because calling `mprotect` with different permissions will split up the VMAs in the kernel, just map everything RWX.