Fix ice tests when librustc-driver is linked dynamically
Running `dump-ice-to-disk`and `short-ice` tests on Linux targeting `x86_64-fortanix-unknown-sgx` platform results in:
```
jenkins@31cf43196355:~/workspace/rust-sgx-ci/Raoul/rust$ cat /home/jenkins/workspace/rust-sgx-ci/Raoul/rust/build/x86_64-unknown-linux-gnu/test/run-make/dump-ice-to-disk/dump-ice-to-disk/*
/home/jenkins/workspace/rust-sgx-ci/Raoul/rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc: error while loading shared libraries: librustc_driver-fa98927b935b2881.so: cannot open shared object file: No such file or directory
/home/jenkins/workspace/rust-sgx-ci/Raoul/rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc: error while loading shared libraries: librustc_driver-fa98927b935b2881.so: cannot open shared object file: No such file or directory
/home/jenkins/workspace/rust-sgx-ci/Raoul/rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc: error while loading shared libraries: librustc_driver-fa98927b935b2881.so: cannot open shared object file: No such file or directory
/home/jenkins/workspace/rust-sgx-ci/Raoul/rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc: error while loading shared libraries: librustc_driver-fa98927b935b2881.so: cannot open shared object file: No such file or directory
```
Setting the LD_LIBRARY_PATH explicitly to `$(HOST_RPATH_DIR)` in these tests Makefiles resolves the issue. The `thumb-none-qemu` and `thumb-none-cortex-m` run-make tests do something similar.
cc: ```@jethrogb``` ```@vn971``` ```@mkaynov```
93d65adb36