gdb: compile with -fcommon explicitly

As described in the comment, gdb relies on some of the linker magic that
happens with -fcommon. I suspect the life expectancy of gdb-in-base is low
enough that this isn't worth spending much time addressing, especially given
the vintage. Hit it with the -fcommon hammer so that it continues to just
work.

MFC after:	3 days

gnu/usr.bin/gdb/Makefile.inc

@@ -47,6 +47,11 @@ CFLAGS+= -I${CNTRB_GDB}/include
 CFLAGS+= -I${CNTRB_BU}/bfd
 CFLAGS+= -I${SYSROOT:U${DESTDIR}}/${INCLUDEDIR}/edit
 
+# Some bits here currently rely on some of the linker-merging magic that happens
+# with -fcommon.  While this is the default right now, explicitly set -fcommon
+# so that it continues to build when the default flips.
+CFLAGS+= -fcommon
+
 GENSRCS+= nm.h tm.h
 
 .if defined(GDB_CROSS_DEBUGGER)