[crypto] Harden hardened_xor function

[nasahlpa]

This PR improves the hardened_xor function.

In the previous version of this function, when passing two shares of a secret variable to it, one share was overwritten with the other share, which leaks.

The new function now performs:
dest = ((x ^ rand) ^ y) ^ rand

This looks like the following in assembly:

20014574:	418c                	lw	a1,0(a1)
20014576:	4118                	lw	a4,0(a0)
20014578:	8db9                	xor	a1,a1,a4
2001457a:	c28c                	sw	a1,0(a3)
2001457c:	4210                	lw	a2,0(a2)
2001457e:	8db1                	xor	a1,a1,a2
20014580:	c28c                	sw	a1,0(a3)
20014582:	4108                	lw	a0,0(a0)
20014584:	8d2d                	xor	a0,a0,a1
20014586:	c288                	sw	a0,0(a3)

The first commit in this PR belongs to #27984

[johannheyszl]

Thanks @nasahlpa this looks nice!

[h-filali]

Thanks Pascal! I checked the last two commits. One share should not overwrite the other anymore. I also compared the loop to the existing hardened_memcpy implementation.
I also left two questions. But this looks good.

[lowrisc-ci]

Backport failed for earlgrey_1.0.0, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin earlgrey_1.0.0
git worktree add -d .worktree/backport-28085-to-earlgrey_1.0.0 origin/earlgrey_1.0.0
cd .worktree/backport-28085-to-earlgrey_1.0.0
git switch --create backport-28085-to-earlgrey_1.0.0
git cherry-pick -x 542beff0988afcfc65f8fb501beed186600a8544 54116843ab4dc031f04e6cfc3e2452fff6980a74 b126c26037688f186230539c47165d4f7fb6b9b3

[lowrisc-ci]

Backport failed for earlgrey_1.0.0, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin earlgrey_1.0.0
git worktree add -d .worktree/backport-28085-to-earlgrey_1.0.0 origin/earlgrey_1.0.0
cd .worktree/backport-28085-to-earlgrey_1.0.0
git switch --create backport-28085-to-earlgrey_1.0.0
git cherry-pick -x 542beff0988afcfc65f8fb501beed186600a8544 54116843ab4dc031f04e6cfc3e2452fff6980a74 b126c26037688f186230539c47165d4f7fb6b9b3