cputlb: Make store_helper less fragile to compiler optimizations

cputlb: Make store_helper less fragile to compiler optimizations

This has no functional change.

The current function structure is:

    inline QEMU_ALWAYSINLINE
    store_memop() {
        switch () {
            ...
        default:
            qemu_build_not_reached();
        }
    }
    inline QEMU_ALWAYSINLINE
    store_helper() {
        ...
        if (span_two_pages_or_io) {
            ...
            helper_ret_stb_mmu();
        }
        store_memop();
    }
    helper_ret_stb_mmu() {
        store_helper();
    }

Whereas GCC will generate an error at compile-time when an always_inline
function is not inlined, Clang does not.  Nor does Clang prioritize the
inlining of always_inline functions.  Both of these are arguably bugs.

Both `store_memop` and `store_helper` need to be inlined and allow
constant propogations to eliminate the `qemu_build_not_reached` call.

However, if the compiler instead chooses to inline helper_ret_stb_mmu
into store_helper, then store_helper is now self-recursive and the
compiler is no longer able to propagate the constant in the same way.

This does not produce at current QEMU head, but was reproducible
at v4.2.0 with `clang-10 -O2 -fexperimental-new-pass-manager`.

The inline recursion problem can be fixed solely by marking
helper_ret_stb_mmu as noinline, so the compiler does not make an
incorrect decision about which functions to inline.

In addition, extract store_helper_unaligned as a noinline subroutine
that can be shared by all of the helpers.  This saves about 6k code
size in an optimized x86_64 build.

Reported-by: Shu-Chun Weng <scw@google.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>