goruntime: split bootstrap symbols into conditionally compiled files

Due to some changes in the runtime init functions between go 1.7 and
later versions (e.g. runtime.modulesInit() defined after go 1.7), the
kernel code that bootstraps the go runtime had to be split into separate
files which are conditionally compiled using +build flags.

src/gopheros/kernel/goruntime/bootstrap.go

@@ -27,27 +27,6 @@ var (
 	prngSeed = 0xdeadc0de
 )
 
-//go:linkname algInit runtime.alginit
-func algInit()
-
-//go:linkname modulesInit runtime.modulesinit
-func modulesInit()
-
-//go:linkname typeLinksInit runtime.typelinksinit
-func typeLinksInit()
-
-//go:linkname itabsInit runtime.itabsinit
-func itabsInit()
-
-//go:linkname mallocInit runtime.mallocinit
-func mallocInit()
-
-//go:linkname mSysStatInc runtime.mSysStatInc
-func mSysStatInc(*uint64, uintptr)
-
-//go:linkname procResize runtime.procresize
-func procResize(int32) uintptr
-
 // initGoPackages is an alias to main.init which recursively calls the init()
 // methods in all imported packages. Unless this function is called, things like
 // package errors will not be properly initialized causing various problems when
@@ -184,7 +163,7 @@ func getRandomData(r []byte) {
 func Init() *kernel.Error {
 	mallocInitFn()
 	algInitFn()       // setup hash implementation for map keys
-	modulesInitFn()   // provides activeModules
+	modulesInitFn()   // provides activeModules (go 1.8+)
 	typeLinksInitFn() // uses maps, activeModules
 	itabsInitFn()     // uses activeModules
 

src/gopheros/kernel/goruntime/bootstrap_go17.go

@@ -0,0 +1,30 @@
+// +build go1.7,!go1.8
+
+package goruntime
+
+import (
+	_ "unsafe" // required for go:linkname
+)
+
+//go:linkname algInit runtime.alginit
+func algInit()
+
+//go:linkname typeLinksInit runtime.typelinksinit
+func typeLinksInit()
+
+//go:linkname itabsInit runtime.itabsinit
+func itabsInit()
+
+//go:linkname mallocInit runtime.mallocinit
+func mallocInit()
+
+//go:linkname mSysStatInc runtime.mSysStatInc
+func mSysStatInc(*uint64, uintptr)
+
+//go:linkname procResize runtime.procresize
+func procResize(int32) uintptr
+
+// modulesInit is defined on go1.8 so just declare an empty
+// stub for go 1.7 to keep the compiler happy.
+func modulesInit() {
+}

src/gopheros/kernel/goruntime/bootstrap_go18+.go

@@ -0,0 +1,28 @@
+// +build go1.8
+
+package goruntime
+
+import (
+	_ "unsafe" // required for go:linkname
+)
+
+//go:linkname algInit runtime.alginit
+func algInit()
+
+//go:linkname modulesInit runtime.modulesinit
+func modulesInit()
+
+//go:linkname typeLinksInit runtime.typelinksinit
+func typeLinksInit()
+
+//go:linkname itabsInit runtime.itabsinit
+func itabsInit()
+
+//go:linkname mallocInit runtime.mallocinit
+func mallocInit()
+
+//go:linkname mSysStatInc runtime.mSysStatInc
+func mSysStatInc(*uint64, uintptr)
+
+//go:linkname procResize runtime.procresize
+func procResize(int32) uintptr