Fix AMD CoMGR loading

[wsmoses]

Ideally should also be forward/backported to other relevant ROCM versions.

HIPRtc currently does not correctly support LLVM plugins. Specifically, symbols defined within LLVM and/or clang are not available by any plugins when they are loaded. This is partially due to the fact that the one location with these symbols (libamd_comgr) does not mark these symbols as being available globally when it is loaded.

This patch is the first of two pieces required to support LLVM plugins on ROCm, the second of which is a change to ensure libamd_comgr marks LLVM/Clang symbols as exported for when it loads the plugin (https://github.com/RadeonOpenCompute/ROCm-CompilerSupport/pull/43).

cc @jedbrown @LeilaGhaffari