Just tried that and I get the same error but at a different spot.

Code
4/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/7.3.0/../../../../x86_64-w64-mingw32/bin/ar.exe: unable to rename '../Archiving libvsystem.a...
Compiling src/mame/video/crt.cpp...
../../../mingw-gcc/bin/x64/Release/mame_mame/libmicrokey.a'; reason: File exists
microkey.make:279: recipe for target '../../../../mingw-gcc/bin/x64/Release/mame_mame/libmicrokey.a' failed
make[2]: *** [../../../../mingw-gcc/bin/x64/Release/mame_mame/libmicrokey.a] Error 1
makefile:724: recipe for target 'microkey' failed
make[1]: *** [microkey] Error 2
Compiling src/mame/drivers/gts3a.cpp...
make[1]: *** Waiting for unfinished jobs....

I used the following commandline:

Code
[MINGW64] C:\msys64\home\mfora\mame>make -j9 avx2=1 NOWERROR=1 USE_NETWORK=1 TOOLS=1 ARCHOPTS=-march=native