all: rename 'arm' to 'tinygo.arm' for Cortex-M

Let the standard library think that it is compiling for js/wasm.

The most correct way of supporting bare metal Cortex-M targets would be
using the 'arm' build tag and specifying no OS or an 'undefined' OS
(perhaps GOOS=noos?). However, there is no build tag for specifying no
OS at all, the closest possible is GOOS=js which makes very few
assumptions.

Sadly GOOS=js also makes some assumptions: it assumes to be running with
GOARCH=wasm. This would not be such a problem, just add js, wasm and arm
as build tags. However, having two GOARCH build tags leads to an error
in internal/cpu: it defines variables for both architectures which then
conflict.

To work around these problems, the 'arm' target has been renamed to
'tinygo.arm', which should work around these problems. In the future, a
GOOS=noos (or similar) should be added which can work with any
architecture and doesn't implement OS-specific stuff.

src/runtime/arch_tinygoarm.go

@@ -0,0 +1,18 @@
+// +build tinygo.arm
+
+package runtime
+
+import (
+	"unsafe"
+)
+
+const GOARCH = "arm"
+
+// The length type used inside strings and slices.
+type lenType uint32
+
+// The bitness of the CPU (e.g. 8, 32, 64).
+const TargetBits = 32
+
+//go:extern _heap_start
+var heapStart unsafe.Pointer