Fix get_sp implementation for s390x (#389)

On s390x we use a biased CFA: the CFA of a function equal the SP
at function entry plus a constant of 160.

Therefore, the current implementation of get_sp, which uses
_Unwind_GetCFA, is not an appropriate way to retrieve the
current SP value on this platform.

Instead, use _Unwind_GetGR to explicitly retrieve the value
of the stack pointer register %r15.

Author: uweigand

src/backtrace/libunwind.rs

@@ -168,7 +168,8 @@ mod uw {
         #[cfg(all(
             not(all(target_os = "android", target_arch = "arm")),
             not(all(target_os = "freebsd", target_arch = "arm")),
-            not(all(target_os = "linux", target_arch = "arm"))
+            not(all(target_os = "linux", target_arch = "arm")),
+            not(all(target_os = "linux", target_arch = "s390x"))
         ))]
         // This function is a misnomer: rather than getting this frame's
         // Canonical Frame Address (aka the caller frame's SP) it
@@ -179,6 +180,17 @@ mod uw {
         pub fn get_sp(ctx: *mut _Unwind_Context) -> libc::uintptr_t;
     }
 
+    // s390x uses a biased CFA value, therefore we need to use
+    // _Unwind_GetGR to get the stack pointer register (%r15)
+    // instead of relying on _Unwind_GetCFA.
+    #[cfg(all(target_os = "linux", target_arch = "s390x"))]
+    pub unsafe fn get_sp(ctx: *mut _Unwind_Context) -> libc::uintptr_t {
+        extern "C" {
+            pub fn _Unwind_GetGR(ctx: *mut _Unwind_Context, index: libc::c_int) -> libc::uintptr_t;
+        }
+        _Unwind_GetGR(ctx, 15)
+    }
+
     // On android and arm, the function `_Unwind_GetIP` and a bunch of others
     // are macros, so we define functions containing the expansion of the
     // macros.