libgcc: fix the handling of return address mangling [PR94891]

libgcc: fix the handling of return address mangling [PR94891]

Mangling, currently only used on AArch64 for return address signing,
is an internal representation that should not be exposed via

  __builtin_return_address return value,
  __builtin_eh_return handler argument,
  _Unwind_DebugHook handler argument.

Note that a mangled address might not even fit into a void *, e.g.
with AArch64 ilp32 ABI the return address is stored as 64bit, so
the mangled return address cannot be accessed via _Unwind_GetPtr.

This patch changes the unwinder hooks as follows:

MD_POST_EXTRACT_ROOT_ADDR is removed: root address comes from
__builtin_return_address which is not mangled.

MD_POST_EXTRACT_FRAME_ADDR is renamed to MD_DEMANGLE_RETURN_ADDR,
it now operates on _Unwind_Word instead of void *, so the hook
should work when return address signing is enabled on AArch64 ilp32.
(But for that __builtin_aarch64_autia1716 should be fixed to operate
on 64bit input instead of a void *.)

MD_POST_FROB_EH_HANDLER_ADDR is removed: it is the responsibility of
__builtin_eh_return to do the mangling if necessary.

2020-07-13  Szabolcs Nagy  <szabolcs.nagy@arm.com>

libgcc/ChangeLog:

	PR target/94891
	* config/aarch64/aarch64-unwind.h (MD_POST_EXTRACT_ROOT_ADDR): Remove.
	(MD_POST_FROB_EH_HANDLER_ADDR): Remove.
	(MD_POST_EXTRACT_FRAME_ADDR): Rename to ...
	(MD_DEMANGLE_RETURN_ADDR): This.
	(aarch64_post_extract_frame_addr): Rename to ...
	(aarch64_demangle_return_addr): This.
	(aarch64_post_frob_eh_handler_addr): Remove.
	* unwind-dw2.c (uw_update_context): Demangle return address.
	(uw_frob_return_addr): Remove.