Use std::string to simplify build_id_to_debug_bfd

Use std::string to simplify build_id_to_debug_bfd

Using std::string here makes the string building simpler thank playing
with char*.  A stack allocation is replaced with heap allocation, but
I don't think this is really performance-critical code.

gdb/ChangeLog:

	* build-id.c (build_id_to_debug_bfd): Use std::string.