x86/retpoline: Create a retpoline thunk array

Stick all the retpolines in a single symbol and have the individual
thunks as inner labels, this should guarantee thunk order and layout.

Previously there were 16 (or rather 15 without rsp) separate symbols and
a toolchain might reasonably expect it could displace them however it
liked, with disregard for their relative position.

However, now they're part of a larger symbol. Any change to their
relative position would disrupt this larger _array symbol and thus not
be sound.

This is the same reasoning used for data symbols. On their own there
is no guarantee about their relative position wrt to one aonther, but
we're still able to do arrays because an array as a whole is a single
larger symbol.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.169659320@infradead.org