userland build: replace -fno-common with ${CFCOMMONFLAG}

This change allows any downstream or otherwise consumer to easily override
the new -fno-common default on a temporary basis without having to hack into
src.sys.mk, and also makes it a bit easier to search for these specific
cases where -fno-common must be overridden with -fcommon or else the build
will fail.

The gdb build, the only program requiring -fcommon on head/, is switched
over as an example usage. It will need it on all branches, so this does not
harm future mergability.

MFC after:	3 days

gnu/usr.bin/gdb/Makefile.inc

@@ -50,7 +50,7 @@ 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
+CFCOMMONFLAG= -fcommon
 
 GENSRCS+= nm.h tm.h
 

share/mk/src.sys.mk

@@ -37,7 +37,8 @@ __postrcconf_${var}:=	${MK_${var}:U-}${WITHOUT_${var}:Uno:Dyes}${WITH_${var}:Uno
 # The following should be removed no earlier than LLVM11 being imported into the
 # tree, to ensure we don't regress the build.  LLVM11 and GCC10 will switch the
 # default over to -fno-common, making this redundant.
-CFLAGS+=	-fno-common
+CFCOMMONFLAG?=	-fno-common
+CFLAGS+=	${CFCOMMONFLAG}
 
 # tempting, but bsd.compiler.mk causes problems this early
 # probably need to remove dependence on bsd.own.mk