all: remove need for -fcommon

Use `extern` where appropriate so that we can remove `-fcommon`
on macOS and others.  On macOS, `-fcommon` generates a number of
linker warnings.

I've tested building on macOS, Linux, FreeBSD, NetBSD, OpenBSD
and illumos.  I am not in a position to test on AIX or other
more esoteric platforms, however.

src/libthread/thread.c

@@ -690,6 +690,7 @@ threadrwakeup(Rendez *r, int all, ulong pc)
 
 static int threadargc;
 static char **threadargv;
+#pragma weak mainstacksize
 int mainstacksize;
 extern int _p9usepwlibrary;	/* getgrgid etc. smash the stack - tell _p9dir just say no */
 static void