ARM64 build on Windows (MSVC) is broken

Not sure we ever supported Windows ARM64 previously, (I guess we didn't?)

[1/6572] Compiling C object subprojects/orc/orc/orc-0.4-0.dll.p/orccpu-arm.c.obj
FAILED: subprojects/orc/orc/orc-0.4-0.dll.p/orccpu-arm.c.obj
"cl" "-Isubprojects\orc\orc\orc-0.4-0.dll.p" "-Isubprojects\orc\orc" "-I..\subprojects\orc\orc" "-Isubprojects\orc" "-I..\subprojects\orc" "/MD" "/nologo" "/showIncludes" "/W2" "/O2" "/Ob1" "/Zi" "-DHAVE_CONFIG_H" "/utf-8" "-DORC_ENABLE_UNSTABLE_API" "-D_GNU_SOURCE" "-DBUILDING_ORC" "/Fdsubprojects\orc\orc\orc-0.4-0.dll.p\orccpu-arm.c.pdb" /Fosubprojects/orc/orc/orc-0.4-0.dll.p/orccpu-arm.c.obj "/c" ../subprojects/orc/orc/orccpu-arm.c
../subprojects/orc/orc/orccpu-arm.c(35): fatal error C1083: Cannot open include file: 'unistd.h': No such file or directory
[4/6572] Compiling C object subprojects/orc/orc/orc-0.4-0.dll.p/orcarm.c.obj
FAILED: subprojects/orc/orc/orc-0.4-0.dll.p/orcarm.c.obj
"cl" "-Isubprojects\orc\orc\orc-0.4-0.dll.p" "-Isubprojects\orc\orc" "-I..\subprojects\orc\orc" "-Isubprojects\orc" "-I..\subprojects\orc" "/MD" "/nologo" "/showIncludes" "/W2" "/O2" "/Ob1" "/Zi" "-DHAVE_CONFIG_H" "/utf-8" "-DORC_ENABLE_UNSTABLE_API" "-D_GNU_SOURCE" "-DBUILDING_ORC" "/Fdsubprojects\orc\orc\orc-0.4-0.dll.p\orcarm.c.pdb" /Fosubprojects/orc/orc/orc-0.4-0.dll.p/orcarm.c.obj "/c" ../subprojects/orc/orc/orcarm.c
../subprojects/orc/orc/orcarm.c(941): error C2296: '+': illegal, left operand has type 'OrcExecutorFunc'