Factor out C++-dependent code

[andreittr]

lib-compiler-rt is relied on by clang/llvm to implement various floating point operations which can come up in C code.
However, lib-compiler-rt also depends on libunwind (and, therefore, an entire C++ runtime) to implement stack unwinding, chiefly in gcc_personality_v0.c. This leads to unpleasant situations such as requiring a C++ runtime to compile musl with complex number support with clang.

This issue proposes splitting up lib-compiler-rt (via Kconfig) into (1) the pure C parts, and (2) the C++ depedent parts. This way one can make use of its arithmetic builtins without pulling in all of C++.