replace VLA with alloca for MSVC

VLA's are C99, but technically a GNU extension in C++. MSVC 2013 chokes on them.
```
/d/code/msys64/home/Tony/julia/deps/libuv/compile cl -nologo -MD -Z7 -TP -EHsc -
DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORM
AT_MACROS -D__STDC_LIMIT_MACROS -DJL_SYSTEM_IMAGE_PATH="../lib/julia/sys.ji" -DJ
ULIA_TARGET_ARCH=x86-64 -DSYSTEM_LLVM -D_WIN32_WINNT=0x0600 -D_GNU_SOURCE -Iflis
p -Isupport -I/d/code/msys64/home/Tony/julia/usr/bin/../include -I/d/code/msys64
/home/Tony/julia/deps/libuv/include -I/d/code/msys64/home/Tony/julia/usr/include
 -DLIBRARY_EXPORTS -c codegen.cpp -o codegen.o
codegen.cpp
d:\code\msys64\home\tony\julia\src\disasm.cpp(143) : error C2057: expected const
ant expression
d:\code\msys64\home\tony\julia\src\disasm.cpp(143) : error C2466: cannot allocat
e an array of constant size 0
d:\code\msys64\home\tony\julia\src\disasm.cpp(143) : error C2133: 'bytes' : unkn
own size
make[2]: *** [codegen.o] Error 2
make[2]: Leaving directory `/d/code/msys64/home/Tony/julia/src'
```

cc @eschnett