Add details about MSVCRT and AVX in README.md

UCRT is gradually growing in popularity, and it wasn't obvious from
README.md that w64devkit is a traditional MSVCRT toolchain.

The AVX issue comes up from time to time, just infrequently enough that
I need to rediscover the solution each time. Documentation in README.md
will be easier to find next time, warns those who haven't been bitten
yet, and documents the mitigation offline. Since it is unlikely to be
fixed any time soon, medium term I should figure out a small patch that
makes GCC only emit unaligned AVX moves. The penalty for unaligned moves
is 0% on newer processors and up to 10% on older ones. That's certainly
better than crashing due to a compilier bug.

https://lemire.me/blog/2012/05/31/data-alignment-for-speed-myth-or-reality/

Author: skeeto

README.md

@@ -1,8 +1,8 @@
 # Portable C, C++, and Fortran Development Kit for x64 and x86 Windows
 
 [w64devkit][] is a Dockerfile that builds from source a small, portable
-development suite for creating C and C++ applications on and for x64
-Windows. See "Releases" for pre-built, ready-to-use kits.
+development suite for creating C and C++ applications on and for x86 and
+x64 Windows. See "Releases" for pre-built, ready-to-use kits.
 
 Included tools:
 
@@ -13,10 +13,10 @@ Included tools:
 * [Vim][vim] : powerful text editor
 * [Universal Ctags][ctags] : source navigation
 
-The toolchain includes pthreads, C++11 threads, and OpenMP. All included
-runtime components are static. **Docker/Podman is not required to use the
-development kit**. It's merely a reliable, clean environment for building
-the kit itself.
+It is an MSVCRT toolchain with pthreads, C++11 threads, and OpenMP. All
+included runtime components are static. **Docker/Podman is not required to
+use the development kit**. It's merely a reliable, clean environment for
+building the kit itself.
 
 ## Build
 
@@ -60,8 +60,7 @@ Then to start an interactive unix shell:
 * Trivial to build from source, meaning it's easy to tweak and adjust any
   part of the kit for your own requirements.
 
-* [Complements Go](https://nullprogram.com/blog/2021/06/29/) for cgo and
-  bootstrapping.
+* [Complements Go][go] for cgo and bootstrapping.
 
 ## Optimized for size
 
@@ -166,16 +165,16 @@ encapsulating the entire development environment, with home directory, on
 removable, even read-only, media. Use a `.profile` in the home directory
 to configure the environment further.
 
-I'd love to include Git, but unfortunately Git's build system doesn't
-quite support cross-compilation. A decent alternative would be
-[Quilt][quilt], but it's written in Bash and Perl.
-
 Neither Address Sanitizer (ASan) nor Thread Sanitizer (TSan) [has been
 ported to Mingw-w64][san] ([also][san2]), but Undefined Behavior Sanitizer
 (UBSan) works perfectly under GDB. With both `-fsanitize=undefined` and
 `-fsanitize-trap`, GDB will [break precisely][break] on undefined
 behavior, and it does not require linking with libsanitizer.
 
+[GCC does not fully support AVX on Windows][avx] and may use aligned moves
+on unaligned addresses. When targeting AVX, consider disabling all aligned
+moves in the assembler: `-Wa,-muse-unaligned-vector-move`.
+
 ## Licenses
 
 When distributing binaries built using w64devkit, your .exe will include
@@ -190,6 +189,7 @@ w64devkit includes the concatenated set of all licenses in the file
 binaries.
 
 
+[avx]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54412
 [bb]: https://frippery.org/busybox/
 [break]: https://nullprogram.com/blog/2022/06/26/
 [bs]: https://www.rdegges.com/2016/i-dont-give-a-shit-about-licensing/
@@ -213,7 +213,6 @@ binaries.
 [lic2]: https://sourceforge.net/p/mingw-w64/mingw-w64/ci/master/tree/mingw-w64-libraries/winpthreads/COPYING
 [make]: https://www.gnu.org/software/make/
 [names]: https://learn.microsoft.com/en-us/cpp/build/reference/decorated-names
-[quilt]: http://savannah.nongnu.org/projects/quilt
 [san]: http://mingw-w64.org/doku.php/contribute#sanitizers_asan_tsan_usan
 [san2]: https://groups.google.com/forum/#!topic/address-sanitizer/q0e5EBVKZT4
 [vim]: https://www.vim.org/