runtime: support panic recovery on Xtensa

Use picolibc setjmp and longjmp to save and restore the complete
Xtensa register-window state at TinyGo panic checkpoints.

Provide the target configuration and window spill helper required by
picolibc. Increment its cache version because Xtensa archives now
include another source file.

compileopts/config.go

@@ -25,7 +25,7 @@ import (
 var libVersions = map[string]int{
 	"musl":         3,
 	"bdwgc":        2,
-	"picolibc":     1,
+	"picolibc":     2,
 	"wasmbuiltins": 1,
 }