FROMLIST: x86, mem: move memmove to out of line assembler

When building ARCH=i386 with CONFIG_LTO_CLANG_FULL=y, it's possible
(depending on additional configs which I have not been able to isolate)
to observe a failure during register allocation:

  error: inline assembly requires more registers than available

when memmove is inlined into tcp_v4_fill_cb() or tcp_v6_fill_cb().

memmove is quite large and probably shouldn't be inlined due to size
alone. A noinline function attribute would be the simplest fix, but
there's a few things that stand out with the current definition:

In addition to having complex constraints that can't always be resolved,
the clobber list seems to be missing %bx and %dx, and possibly %cl. By
using numbered operands rather than symbolic operands, the constraints
are quite obnoxious to refactor.

Having a large function be 99% inline asm is a code smell that this
function should simply be written in stand-alone out-of-line assembler.
That gives the opportunity for other cleanups like fixing the
inconsistent use of tabs vs spaces and instruction suffixes, and the
label 3 appearing twice.  Symbolic operands and local labels would
provide this code with a fresh coat of paint.

Moving this to out of line assembler guarantees that the
compiler cannot inline calls to memmove.

This has been done previously for 64b:
commit 9599ec04 ("x86-64, mem: Convert memmove() to assembly file
and fix return value bug")

Bug: 247605214
Link: https://lore.kernel.org/llvm/20220927210248.3950201-1-ndesaulniers@google.com/


Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: I5fde7a76d915c20a594dd9e0d409015855e731b2