Consider rewriting Sancus LLVM pass as a GCC plugin

[jovanbulck]

Currently, sancus-cc is implemented as an LLVM pass that prepares SMs for sancus-ld by:

1. Placing SM_ENTRY/SM_FUNC code and SM_DATA data in respectively .text.sm.name.text and .sm.name.data ELF sections

2. Replacing inter-protection domain function calls with custom asm stubs that call to the corresponding sm_entry/exit/verify stubs.

3. Creating an entry function table with (adrs, args_len, ret_len) for use by sm_entry.

Since the MSP430 LLVM back-end generates inferior code, we should investigate the feasibility of doing the above in a GCC plugin. We then could benefit from the clean, stable, and fast code generated by MSP430GCC.

Note that sancus-cc is our only dependency on LLVM/Clang, for sancus-ld already uses MSP430GCC.

[jovanbulck]

@pmaene also had an idea of implementing the entire Sancus pass inside the sancus-ld Python script by:

1. Annotating C source code with the GCC section attrribute
2. Patching ELF relocation target addresses to instrument inter-SM calls

A bit hacky of course, but to be considered if we ever need it..

[jovanbulck]

Another quick note: we might also want to consider using the ROSE source-to-source compiler infrastructure to implement a dedicated precompiler that can take care of some of the stuff above.

[jovanbulck]

We could possibly also develop a GCC plugin in MELT or GCC-Python