nat/amd64-linux-siginfo.c: Remove typedefs

nat/amd64-linux-siginfo.c: Remove typedefs

Since GDB is written in C++ now, we don't need struct/union typedefs
any more.  Remove them from nat/amd64-linux-siginfo.c.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* nat/amd64-linux-siginfo.c (union nat_sigval): Rename to ...
	(nat_sigval_t): ... this and remove typedef of same name.
	(struct nat_siginfo): Rename to ...
	(nat_siginfo_t): ... this and remove typedef of same name.
	(struct compat_sigval): Rename to ...
	(compat_sigval_t): ... this and remove typedef of same name.
	(struct compat_siginfo): Rename to ...
	(compat_siginfo_t): ... this and remove typedef of same name.
	(struct compat_x32_siginfo): Rename to ...
	(compat_x32_siginfo_t): ... this and remove typedef of same name.
	(amd64_linux_siginfo_fixup_common): Adjust.