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/baremetal.go

@@ -0,0 +1,30 @@
+// +build avr cortexm tinygo.riscv
+
+package runtime
+
+import (
+	"unsafe"
+)
+
+//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))
+)