package/go: use host compiler when go-bootstrap unsupported

All Go compiler versions > 1.4.x (old) are written in Go, and require a existing
compiled Go version to use to build from source.

https://golang.org/doc/install/source#bootstrapFromSource

The process for "bootstrapping" the Go compiler in Buildroot is:

1. Compile a C/C++ cross-compiler (gcc) as the host toolchain.
2. Build go-bootstrap-stage1 (which is Go 1.4.x and written in C)
3. Build go-bootstrap-stage2 (which is Go 1.19.x and written in Go)
3. Build go 1.20 (written in Go) using go-bootstrap-stage2.

go-bootstrap-stage1 does not work on 64-bit arm. The Go 1.4.x bootstrap compiler
is compatible with x86, x86_64, and arm (32 bit) only.

This patch adds a fallback to require a host Go compiler to build host-go when
BR2_PACKAGE_HOST_GO_BOOTSTRAP_STAGE2_ARCH_SUPPORTS is not set.

Signed-off-by: Christian Stewart <christian@paral.in>

---

changes prior to inclusion in this series:

 - thanks Thomas for the review & suggestions
 - added NEEDS_HOST_GO boolean
 - added dependency checks to support/dependencies/dependencies.sh
 - removed unnecessary changes to go-bootstrap package
 - add dependency on toolchain if Cgo is enabled
 - updates for go1.20
 - updates for go-bootstrap-stage{1,2}

changes from v1 -> v2:

 - remove whitespace fix in bootstrap stage2

Signed-off-by: Christian Stewart <christian@paral.in>

Config.in

@@ -58,6 +58,10 @@ config BR2_HOST_GCC_AT_LEAST_9
 # When adding new entries above, be sure to update
 # the HOSTCC_MAX_VERSION variable in the Makefile.
 
+# Hidden boolean selected if bootstrapping Go w/ GCC is not supported.
+config BR2_NEEDS_HOST_GO
+	bool
+
 # Hidden boolean selected by packages in need of Java in order to build
 # (example: kodi)
 config BR2_NEEDS_HOST_JAVA

package/go/Config.in.host

@@ -30,4 +30,4 @@ config BR2_PACKAGE_HOST_GO_TARGET_CGO_LINKING_SUPPORTS
 config BR2_PACKAGE_HOST_GO_HOST_ARCH_SUPPORTS
 	bool
 	default y
-	depends on BR2_PACKAGE_HOST_GO_BOOTSTRAP_STAGE2_ARCH_SUPPORTS
+	select BR2_NEEDS_HOST_GO if !BR2_PACKAGE_HOST_GO_BOOTSTRAP_STAGE2_ARCH_SUPPORTS

package/go/go.mk

@@ -12,7 +12,6 @@ GO_LICENSE = BSD-3-Clause
 GO_LICENSE_FILES = LICENSE
 GO_CPE_ID_VENDOR = golang
 
-HOST_GO_DEPENDENCIES = host-go-bootstrap-stage2
 HOST_GO_GOPATH = $(HOST_DIR)/share/go-path
 HOST_GO_HOST_CACHE = $(HOST_DIR)/share/host-go-cache
 HOST_GO_ROOT = $(HOST_DIR)/lib/go
@@ -109,6 +108,11 @@ else # !BR2_PACKAGE_HOST_GO_TARGET_ARCH_SUPPORTS
 HOST_GO_CGO_ENABLED = 1
 endif # BR2_PACKAGE_HOST_GO_TARGET_ARCH_SUPPORTS
 
+ifeq ($(HOST_GO_CGO_ENABLED),1)
+# For cgo support the toolchain needs to be available.
+HOST_GO_DEPENDENCIES += toolchain
+endif
+
 # For the convenience of host golang packages
 HOST_GO_HOST_ENV = \
 	$(HOST_GO_COMMON_ENV) \
@@ -126,7 +130,6 @@ HOST_GO_HOST_ENV = \
 HOST_GO_MAKE_ENV = \
 	GO111MODULE=off \
 	GOCACHE=$(HOST_GO_HOST_CACHE) \
-	GOROOT_BOOTSTRAP=$(HOST_GO_BOOTSTRAP_STAGE2_ROOT) \
 	GOROOT_FINAL=$(HOST_GO_ROOT) \
 	GOROOT="$(@D)" \
 	GOBIN="$(@D)/bin" \
@@ -136,6 +139,13 @@ HOST_GO_MAKE_ENV = \
 	CGO_ENABLED=$(HOST_GO_CGO_ENABLED) \
 	$(HOST_GO_CROSS_ENV)
 
+# Use the Go compiler bootstrapped by Buildroot if available.
+# Otherwise, use the host Go compiler.
+ifeq ($(BR2_PACKAGE_HOST_GO_BOOTSTRAP_STAGE2_ARCH_SUPPORTS),y)
+HOST_GO_DEPENDENCIES += host-go-bootstrap-stage2
+HOST_GO_MAKE_ENV += GOROOT_BOOTSTRAP=$(HOST_GO_BOOTSTRAP_STAGE2_ROOT)
+endif
+
 define HOST_GO_BUILD_CMDS
 	cd $(@D)/src && \
 		$(HOST_GO_MAKE_ENV) ./make.bash $(if $(VERBOSE),-v)

support/dependencies/dependencies.sh

@@ -217,6 +217,10 @@ if grep -q ^BR2_NEEDS_HOST_UTF8_LOCALE=y $BR2_CONFIG ; then
 	fi
 fi
 
+if grep -q ^BR2_NEEDS_HOST_GO=y $BR2_CONFIG ; then
+	check_prog_host "go"
+fi
+
 if grep -q ^BR2_NEEDS_HOST_JAVA=y $BR2_CONFIG ; then
 	check_prog_host "java"
 	JAVA_GCJ=$(java -version 2>&1 | grep gcj)