I was tring to freeze some libraries into a mingw windows build today, forgetting there was viper code in some of them.
There's an odd compiler error though that took me way too long to figure out was related to the viper functions, so thought it was worth documenting here:

micropython/ports/windows$ make CROSS_COMPILE=i686-w64-mingw32- FROZEN_MANIFEST=manifest.py
...
CC build/frozen_content.c
/tmp/ccm1KbYo.s: Assembler messages:
/tmp/ccm1KbYo.s:225: Warning: Ignoring changed section attributes for .text
/tmp/ccm1KbYo.s:225: Error: junk at end of line, first unrecognized character is `,'
/tmp/ccm1KbYo.s:270: Warning: Ignoring changed section attributes for .text
/tmp/ccm1KbYo.s:270: Error: junk at end of line, first unrecognized character is `,'

building from current master (d42cba0d22c) with

$ i686-w64-mingw32-gcc -v
Using built-in specs.
COLLECT_GCC=i686-w64-mingw32-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-w64-mingw32/9.3-win32/lto-wrapper
Target: i686-w64-mingw32
Configured with: ../../src/configure --build=x86_64-linux-gnu --prefix=/usr --includedir='/usr/include' --mandir='/usr/share/man' --infodir='/usr/share/info' --sysconfdir=/etc --localstatedir=/var --disable-silent-rules --libdir='/usr/lib/x86_64-linux-gnu' --libexecdir='/usr/lib/x86_64-linux-gnu' --disable-maintainer-mode --disable-dependency-tracking --prefix=/usr --enable-shared --enable-static --disable-multilib --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --libdir=/usr/lib --enable-libstdcxx-time=yes --with-tune=generic --with-headers=/usr/i686-w64-mingw32/include --enable-version-specific-runtime-libs --enable-fully-dynamic-string --enable-libgomp --enable-languages=c,c++,fortran,objc,obj-c++,ada --enable-lto --enable-threads=win32 --program-suffix=-win32 --program-prefix=i686-w64-mingw32- --target=i686-w64-mingw32 --with-as=/usr/bin/i686-w64-mingw32-as --with-ld=/usr/bin/i686-w64-mingw32-ld --enable-libatomic --enable-libstdcxx-filesystem-ts=yes --enable-dependency-tracking
Thread model: win32
gcc version 9.3-win32 20200320 (GCC)

details:

micropython/ports/windows$ cat manifest.py
freeze("../../tests/micropython")

micropython/ports/windows$ make CROSS_COMPILE=i686-w64-mingw32- FROZEN_MANIFEST=manifest.py CFLAGS_EXTRA='-save-temps'
...
frozen_content.s:6875: Error: junk at end of line, first unrecognized character is `,'
make: *** [../../py/mkrules.mk:77: build/build/frozen_content.o] Error 1

src/micropython/ports/windows$ awk 'NR==6872,NR==6878{print NR" "$0}' frozen_content.s
6872    .word   4086
6873    .word   -28951
6874    .word   1
6875    .section        .text,"ax",@progbits # ,"dr"
6876    .align 32
6877 _fun_data_native_const_intbig__lt_module_gt__f:
6878    .ascii "USATAUH\203\354"

I gather for actual viper support on windows we'll need a finished version of https://github.com/micropython/micropython/pull/4699 but this error looks like it'll need a fix similar to https://github.com/micropython/micropython/pull/2851 based on the issues that PR closed?