xtensa: add asm-prototypes.h

Move assembly source prototypes from xtensa_ksyms.c to
asm/asm-prototypes.h, move corresponding EXPORT_SYMBOLs to the assembly
sources and enable HAVE_ASM_MODVERSIONS for xtensa.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

xtensa: add __bswap{si,di}2 helpers

gcc-13 may generate calls for __bswap{si,di}2. This breaks the kernel
build when optimization for size is selected. Add __bswap{si,di}2
helpers to fix that.

Cc: stable@vger.kernel.org
Fixes: 19c5699f9aff ("xtensa: don't link with libgcc")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>