package/go: cgo for the target needs the toolchain

Building go with cgo support needs to build some .c files to generate target
support code, and thus calls the cross C compiler, which is failing when the
toolchain is not built before host-go:

    >>> host-go 1.21.1 Building
    cd .../build/host-go-1.21.1/src && GO111MODULE=off GOCACHE=.../per-package/host-go/host/share/host-go-cache GOROOT_BOOTSTRAP=.../per-package/host-go/host/lib/go-1.19.11 GOROOT_FINAL=.../per-package/host-go/host/lib/go GOROOT=".../build/host-go-1.21.1" GOBIN=".../build/host-go-1.21.1/bin" GOOS=linux CC=/usr/bin/gcc CXX=/usr/bin/g++ CGO_ENABLED=1 CC_FOR_TARGET=".../per-package/host-go/host/bin/arm-linux-gcc" CXX_FOR_TARGET=".../per-package/host-go/host/bin/arm-linux-g++" GOOS="linux" GOARCH=arm  GOARM=6 GO_ASSUME_CROSSCOMPILING=1 ./make.bash
    Building Go cmd/dist using .../per-package/host-go/host/lib/go-1.19.11.  (go1.19.11 linux/amd64)
    go tool dist: cannot invoke C compiler [".../per-package/host-go/host/bin/arm-linux-gcc"]: fork/exec .../per-package/host-go/host/bin/arm-linux-gcc: no such file or directory

    Go needs a system C compiler for use with cgo.
    To set a C compiler, set CC=the-compiler.
    To disable cgo, set CGO_ENABLED=0.

This happens systematically with PPD, and happens without PPD when
host-go is explicitly built (by running: "make host-go").

Since only CGO support needs to compile C files, only add the toolchain
dependency in that case.

When the target is not supported by go, then there is obviously no need
to depend on the toolchain (even if we unconditionally enable cgo
support in only-for-the-host host-go).

Signed-off-by: Christian Stewart <christian@aperture.us>
[yann.morin@orange.com:
  - only add the toolchain dependency for target cgo
  - reword commit log
]
Signed-off-by: Yann E. MORIN <yann.morin@orange.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Anisse Astier <anisse@astier.eu>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 1a44f9242c960dcb114f60674043c8044c71c2c0)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/go/go.mk

@@ -89,6 +89,7 @@ HOST_GO_TARGET_ENV = \
 # any target package needing cgo support must include
 # 'depends on BR2_TOOLCHAIN_HAS_THREADS' in its config file.
 ifeq ($(BR2_TOOLCHAIN_HAS_THREADS),y)
+HOST_GO_DEPENDENCIES += toolchain
 HOST_GO_CGO_ENABLED = 1
 else
 HOST_GO_CGO_ENABLED = 0