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.

src/runtime/arch_xtensa.go

@@ -11,7 +11,9 @@ const zeroSizeAllocPtr uintptr = 16 // part of early flash: partition table, etc
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 32
 
-const deferExtraRegs = 0
+// Xtensa's windowed ABI uses a 17-word jmp_buf. JumpSP and JumpPC provide the
+// first two words.
+const deferExtraRegs = 15
 
 const callInstSize = 3 // "callx0 someFunction" (and similar) is 3 bytes
 

src/runtime/asm_xtensa.S

@@ -0,0 +1,34 @@
+#ifdef __XTENSA_WINDOWED_ABI__
+.section .text.__xtensa_libgcc_window_spill,"ax",@progbits
+.global __xtensa_libgcc_window_spill
+.type __xtensa_libgcc_window_spill, %function
+__xtensa_libgcc_window_spill:
+    entry sp, 16
+    movi a6, 15
+    call4 .Lspill_windows
+    retw
+.size __xtensa_libgcc_window_spill, .-__xtensa_libgcc_window_spill
+
+.balign 4
+.Lspill_windows:
+    entry sp, 16
+    beqz a2, .Lspill_done
+    addi a2, a2, -1
+    mov a6, a2
+    call4 .Lspill_windows
+.Lspill_done:
+    retw
+#endif
+
+.section .text.tinygo_longjmp,"ax",@progbits
+.global tinygo_longjmp
+.type tinygo_longjmp, %function
+tinygo_longjmp:
+#ifdef __XTENSA_WINDOWED_ABI__
+    // longjmp's entry rotates a10/a11 into its a2/a3 arguments.
+    movi a11, 1
+#else
+    movi a3, 1
+#endif
+    j longjmp
+.size tinygo_longjmp, .-tinygo_longjmp