🚩Flag day for the compiler / RTOS!🚩

The first public version of the CHERIoT ABI used different relocation types for read-only and read-write globals. These are accessed via either pcc or cgp (all memory accesses must be via some authorising capability). The former requires an auipcc instruction, the latter may use auicgp for large displacements but usually applies a linker relaxation and ends up using cgp directly.