go: Improve PRINT_PLIST_AWK patterns and avoid possible double definition

- Move GO_PLATFORM definition in lang/go/version.mk in order that also lang/go*
  packages can (re)use it
- Change PRINT_PLIST_AWK pattern that replace all ${GO_PLATFORM} and apply it
  only when ${GO_PLATFORM} is a directory (between two "/").  There are only
  3 exceptions to that in lang/go14.
  Move it to version.mk so lang/go* PLIST can be mostly automatically
  generated.

These changes should help to avoid (most) manual editing of
lang/go*/PLIST.

Index: go-package.mk
===================================================================
RCS file: /cvsroot/pkgsrc/lang/go/go-package.mk,v
retrieving revision 1.19
diff -u -p -r1.19 go-package.mk
--- go-package.mk	14 Oct 2018 18:15:34 -0000	1.19
+++ go-package.mk	23 Oct 2018 15:21:47 -0000
@@ -62,12 +62,8 @@ USE_TOOLS+=		pax
 
 BUILD_DEPENDS+=		${GO_PACKAGE_DEP}
 
-GO_PLATFORM=		${LOWER_OPSYS}_${GOARCH}
 GOTOOLDIR=		go${GOVERSSUFFIX}/pkg/tool/${GO_PLATFORM}
 
-PRINT_PLIST_AWK+=	/${GO_PLATFORM}/ { gsub(/${GO_PLATFORM}/, \
-			"$${GO_PLATFORM}"); \
-			print; next; }
 PRINT_PLIST_AWK+=	/^@pkgdir bin$$/ { next; }
 PRINT_PLIST_AWK+=	/^@pkgdir gopkg$$/ { next; }
 
Index: version.mk
===================================================================
RCS file: /cvsroot/pkgsrc/lang/go/version.mk,v
retrieving revision 1.46
diff -u -p -r1.46 version.mk
--- version.mk	2 Oct 2018 18:19:56 -0000	1.46
+++ version.mk	23 Oct 2018 15:21:47 -0000
@@ -44,5 +44,8 @@ GOOPT=		GOARM=6
 .elif ${MACHINE_ARCH} == "earmv7hf"
 GOOPT=		GOARM=7
 .endif
-PLIST_SUBST+=	GO_PLATFORM=${LOWER_OPSYS:Q}_${GOARCH:Q} GOARCH=${GOARCH:Q}
+GO_PLATFORM=	${LOWER_OPSYS}_${GOARCH}
+PLIST_SUBST+=	GO_PLATFORM=${GO_PLATFORM:Q} GOARCH=${GOARCH:Q}
 PLIST_SUBST+=	GOCHAR=${GOCHAR:Q}
+
+PRINT_PLIST_AWK+=	{ sub("/${GO_PLATFORM}/", "/$${GO_PLATFORM}/") }