parisc/unaligned: Rewrite 64-bit inline assembly of emulate_ldd()

Convert to use real temp variables instead of clobbering processor
registers. This aligns the 64-bit inline assembly code with the 32-bit
assembly code which was rewritten with commit 427c1073
("parisc/unaligned: Rewrite 32-bit inline assembly of emulate_ldd()").

While at it, fix comment in 32-bit rewrite code. Temporary variables are
now used for both 32-bit and 64-bit code, so move their declarations
to the function header.

No functional change intended.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org # v6.0+
Signed-off-by: Helge Deller <deller@gmx.de>