[kernel][dev][gic] convert C to C++

Fairly minimal logical for conversion
Reduce the amount of CDECLS for internal routines
clang-format result

Largest change was the removal of a shadowed register in gicv2 which was
apparently unused.

Tested: validated that both gicv2 and gicv3 work in qemu

Change-Id: Ic5be6eec96b2fb02693f90dfa7eac9926a1c57ae