aarch64: Fix _dl_tlsdesc_dynamic unwind for pac-ret (BZ 32612)

aarch64: Fix _dl_tlsdesc_dynamic unwind for pac-ret (BZ 32612)

When libgcc is built with pac-ret, it requires to autenticate the
unwinding frame based on CFI information.  The _dl_tlsdesc_dynamic
uses a custom calling convention, where it is responsible to save
and restore all registers it might use (even volatile).

The pac-ret support added by 1be3d6eb823d8b952fa54b7bbc90cbecb8981380
was added only on the slow-path, but the fast path also adds DWARF
Register Rule Instruction (cfi_adjust_cfa_offset) since it requires
to save/restore some auxiliary register.  It seems that this is not
fully supported neither by libgcc nor AArch64 ABI [1].

Instead, move paciasp/autiasp to function prologue/epilogue to be
used on both fast and slow paths.

I also corrected the _dl_tlsdesc_dynamic comment description, it was
copied from i386 implementation without any adjustment.

Checked on aarch64-linux-gnu with a toolchain built with
--enable-standard-branch-protection on a system with pac-ret
support.

[1]  https://github.com/ARM-software/abi-aa/blob/main/aadwarf64/aadwarf64.rst#id1

Reviewed-by: Yury Khrustalev <yury.khrustalev@arm.com>