Add platform-specific fixes file

Create a new file, platform.h, for platform-specific hacks

for MSVC, this includes defining strncasecmp to _stricmp and
strdup to _strdup, among other things like defining missing
stat macros

Change some things not supported in MSVC, like _Static_assert,
to their counterparts (in this case, static_assert)

Replace usage of VLAs with malloc and free

Update getopt_long and use the getopt implementation from musl
on Windows.

Use comments to show which functions from platform.h are being used

src/asm/output.c

@@ -30,6 +30,7 @@
 #include "extern/err.h"
 
 #include "linkdefs.h"
+#include "platform.h" // strdup
 
 struct Patch {
 	char tzFilename[_MAX_PATH + 1];