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_tinygoriscv.go

@@ -0,0 +1,19 @@
+// +build tinygo.riscv
+
+package runtime
+
+import "device/riscv"
+
+const GOARCH = "arm" // riscv pretends to be arm
+
+// The bitness of the CPU (e.g. 8, 32, 64).
+const TargetBits = 32
+
+// Align on word boundary.
+func align(ptr uintptr) uintptr {
+	return (ptr + 3) &^ 3
+}
+
+func getCurrentStackPointer() uintptr {
+	return riscv.ReadRegister("sp")
+}