lib/crypto: mips/chacha: Fix clang build and remove unneeded byteswap

The MIPS32r2 ChaCha code has never been buildable with the clang
assembler.  First, clang doesn't support the 'rotl' pseudo-instruction:

    error: unknown instruction, did you mean: rol, rotr?

Second, clang requires that both operands of the 'wsbh' instruction be
explicitly given:

    error: too few operands for instruction

To fix this, align the code with the real instruction set by (1) using
the real instruction 'rotr' instead of the nonstandard pseudo-
instruction 'rotl', and (2) explicitly giving both operands to 'wsbh'.

To make removing the use of 'rotl' a bit easier, also remove the
unnecessary special-casing for big endian CPUs at
.Lchacha_mips_xor_bytes.  The tail handling is actually
endian-independent since it processes one byte at a time.  On big endian
CPUs the old code byte-swapped SAVED_X, then iterated through it in
reverse order.  But the byteswap and reverse iteration canceled out.

Tested with chacha20poly1305-selftest in QEMU using "-M malta" with both
little endian and big endian mips32r2 kernels.

Fixes: 49aa7c00eddf ("crypto: mips/chacha - import 32r2 ChaCha code from Zinc")
Cc: stable@vger.kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505080409.EujEBwA0-lkp@intel.com/
Link: https://lore.kernel.org/r/20250619225535.679301-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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