fix issues with gdb 8.0

fix issues with gdb 8.0

GDB 8.0 is compiled and linked with g++, but the
linking of static targets (f.e. coldfire) fails,
without declaring the functions in thread_db.h
extern C.

The compilation of gdb errors out with:
thread-db.o: In function `thread_db_init()':
thread-db.c:(.text+0x5b6): undefined reference to `td_ta_new(ps_prochandle*, td_thragent**)'
thread-db.c:(.text+0x61e): undefined reference to `td_thr_get_info(td_thrhandle const*, td_thrinfo*)'
thread-db.c:(.text+0x632): undefined reference to `td_symbol_list()'
..