aarch64: add struct _aarch64_ctx to signal.h
aarch64: add struct _aarch64_ctx to signal.h

The unwind code in libgcc uses this type for unwinding across signal
handlers. On aarch64 the kernel may place a sequence of structs on the
signal stack on top of the ucontext to provide additional information.
The unwinder only needs the header, but added all the types the kernel
currently defines for this mechanism because they are part of the uapi.

chardi/musl,chardi/musl,chardi/musl