build: use -muse-unaligned-vector-move for Windows

We currently work around a longstanding GCC issue with aligned vector
instructions, in our release builds, by patching the behaviour we want
into GCC (see discussion in #24736).

A new option now exists in the binutils assembler,
`-muse-unaligned-vector-move`, which should also achieve the behaviour
we want (at least for our code). This was added in the 2.38 release,
see
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=c8480b58e1968f209b6365af7422678f348222c2.
```bash
x86: Add -muse-unaligned-vector-move to assembler

Unaligned load/store instructions on aligned memory or register are as
fast as aligned load/store instructions on modern Intel processors.  Add
a command-line option, -muse-unaligned-vector-move, to x86 assembler to
encode encode aligned vector load/store instructions as unaligned
vector load/store instructions.
```

Even if we introduce this option into our build system, we'll have to
maintain our GCC patching, as we want all code that ends up in the
binary, to avoid these instructions. However, there may be some value in
adding the option, as it could be an improvement for someone building
(bitcoind.exe) with an unpatched compiler.

Author: fanquake

configure.ac

@@ -725,6 +725,10 @@ case $host in
 
      dnl We require Windows 7 (NT 6.1) or later
      AX_CHECK_LINK_FLAG([-Wl,--major-subsystem-version -Wl,6 -Wl,--minor-subsystem-version -Wl,1], [CORE_LDFLAGS="$CORE_LDFLAGS -Wl,--major-subsystem-version -Wl,6 -Wl,--minor-subsystem-version -Wl,1"], [], [$LDFLAG_WERROR])
+
+     dnl Avoid the use of aligned vector instructions when building for Windows.
+     dnl See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54412.
+     AX_CHECK_COMPILE_FLAG([-Wa,-muse-unaligned-vector-move], [CORE_CXXFLAGS="$CORE_CXXFLAGS -Wa,-muse-unaligned-vector-move"], [], [$CXXFLAG_WERROR])
      ;;
   *darwin*)
      TARGET_OS=darwin