Add UNWIND_DATA_REG.

Author: reitermarkus

src/libpanic_unwind/gcc.rs

@@ -117,6 +117,9 @@ const UNWIND_DATA_REG: (i32, i32) = (0, 1); // R0, R1
 #[cfg(target_arch = "riscv64")]
 const UNWIND_DATA_REG: (i32, i32) = (10, 11); // x10, x11
 
+#[cfg(target_arch = "xtensa")]
+const UNWIND_DATA_REG: (i32, i32) = (2, 3); // A2, A3
+
 // The following code is based on GCC's C and C++ personality routines.  For reference, see:
 // https://github.com/gcc-mirror/gcc/blob/master/libstdc++-v3/libsupc++/eh_personality.cc
 // https://github.com/gcc-mirror/gcc/blob/trunk/libgcc/unwind-c.c