gdb: Drop SIGRTMIN definition patch

This has been already fixed in gdb via
https://sourceware.org/pipermail/gdb-patches/2015-October/128532.html



and W_STOPCODE is fixed by including gdbsupport/gdb_wait.h in nat/linux-nat.h

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>