runtime: print the address where a panic happened

This is not very useful in itself, but makes it possible to detect this
address in the output. See the next commit.

This adds around 50 bytes to each binary (except for AVR and wasm). This
is unfortunate, but I think this feature is quite useful still.
A future enhancement might be to create a build tag for extended panic
information that's not set by default.

src/runtime/arch_tinygowasm.go

@@ -13,6 +13,8 @@ const TargetBits = 32
 
 const deferExtraRegs = 0
 
+const callInstSize = 1 // unknown and irrelevant (llvm.returnaddress doesn't work), so make something up
+
 //go:extern __heap_base
 var heapStartSymbol [0]byte