Build gcc4mbed.c as C and not C++.

This was a stragler left over from when all .c files were built as C++ to be
the same as the older mbed online compiler.
Removed __aeabi_unwind_cpp_pr0/1/2() functions from gcc4mbed.c .

These are no longer required with latest newlib nano libraries to help reduce
output binary size and can lead to link errors when other routines from the
same object file in GCC's C++ library are pulled in.