The new stack validator in objdump doesn't like directly assigning r11
to rsp, warning with something like:

warning: objtool: chacha20_4block_xor_ssse3()+0xa: unsupported stack pointer realignment
warning: objtool: chacha20_8block_xor_avx2()+0x6: unsupported stack pointer realignment

This fixes things up to use code similar to gcc's DRAP register, so that
objdump remains happy.
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>
Fixes: baa41469 ("objtool: Implement stack validation 2.0")
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>