Review comments: Remove unneeded `extern "C"`.

The declaration in unwind.h has it already.

(The `extern "C"`s for __aeabi_unwind_cpp_pr* stay around for now, since
these are declared in Registers.hpp, which Unwind-EHABI.cpp doesn't
include yet.)

Change-Id: Ica20af68bf1dae57ae23a3dc3a34d001aedd579e