Split off functionality that depends on libunwind

Split off functionality that depends on libunwind

Previously lib-compiler-rt depended on libunwind as it pulled in
<unwind.h>; this dependency however is isolated in gcc_personality_v0.c.
This change makes gcc personality an optional part of lib-compiler-rt,
allowing it to be built without an entire supporting C++ runtime.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #14