bfd: fix build with C23

Starting in C23, strchr and strrchr will return const char *, if fed a
const char *.  This means that several files in the BFD directory will
fail to build as they are assigning the return of those functions to a
char *.

Fix this by const-ifying several variables. The only place where that
wasn't just that was in targets.c, where a variable was being used in
subsequent strrchr invocations to change the underlying string, so a new
variable had to be introduced.

No user-visible change should happen after this commit.