Adjust frame IP in backtraces relative to image base for SGX target This is followup to https://github.com/rust-lang/backtrace-rs/pull/566. The backtraces printed by `panic!` or generated by `std::backtrace::Backtrace` in SGX target are not usable. The frame addresses need to be relative to image base address so they can be used for symbol resolution. Here's an example panic backtrace generated before this change: ``` $ cargo r --target x86_64-fortanix-unknown-sgx ... stack backtrace: 0: 0x7f8fe401d3a5 - <unknown> 1: 0x7f8fe4034780 - <unknown> 2: 0x7f8fe401c5a3 - <unknown> 3: 0x7f8fe401d1f5 - <unknown> 4: 0x7f8fe401e6f6 - <unknown> ``` Here's the same panic after this change: ``` $ cargo +stage1 r --target x86_64-fortanix-unknown-sgx stack backtrace: 0: 0x198bf - <unknown> 1: 0x3d181 - <unknown> 2: 0x26164 - <unknown> 3: 0x19705 - <unknown> 4: 0x1ef36 - <unknown> ``` cc `@jethrogb` and `@workingjubilee`