all: add HiFive1 rev B board with RISC-V architecture

This page has been a big help in adding support for this new chip:
https://wiki.osdev.org/HiFive-1_Bare_Bones

src/runtime/arch_cortexm.go

@@ -3,8 +3,6 @@
 package runtime
 
 import (
-	"unsafe"
-
 	"device/arm"
 )
 
@@ -13,29 +11,6 @@ const GOARCH = "arm"
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 32
 
-//go:extern _heap_start
-var heapStartSymbol unsafe.Pointer
-
-//go:extern _heap_end
-var heapEndSymbol unsafe.Pointer
-
-//go:extern _globals_start
-var globalsStartSymbol unsafe.Pointer
-
-//go:extern _globals_end
-var globalsEndSymbol unsafe.Pointer
-
-//go:extern _stack_top
-var stackTopSymbol unsafe.Pointer
-
-var (
-	heapStart    = uintptr(unsafe.Pointer(&heapStartSymbol))
-	heapEnd      = uintptr(unsafe.Pointer(&heapEndSymbol))
-	globalsStart = uintptr(unsafe.Pointer(&globalsStartSymbol))
-	globalsEnd   = uintptr(unsafe.Pointer(&globalsEndSymbol))
-	stackTop     = uintptr(unsafe.Pointer(&stackTopSymbol))
-)
-
 // Align on word boundary.
 func align(ptr uintptr) uintptr {
 	return (ptr + 3) &^ 3