test: Use set sysroot for more NDK compatibility

Recent versions of the Android NDK no longer ship debuggers like
`arm-linux-androideabi-gdb`, but instead one prebuilt binary `gdb`. We can
symlink this into place at least to get our detection still working, but it now
needs to be told what the sysroot is so it can correctly do... something. Long
story short, tests didn't pass with this change and after this change they pass.

src/tools/compiletest/src/runtest.rs

@@ -433,9 +433,15 @@ actual:\n\
 
                 cmds = cmds.replace("run", "continue");
 
+                let tool_path = match self.config.android_cross_path.to_str() {
+                    Some(x) => x.to_owned(),
+                    None => self.fatal("cannot find android cross path")
+                };
+
                 // write debugger script
                 let mut script_str = String::with_capacity(2048);
                 script_str.push_str(&format!("set charset {}\n", Self::charset()));
+                script_str.push_str(&format!("set sysroot {}\n", tool_path));
                 script_str.push_str(&format!("file {}\n", exe_file.to_str().unwrap()));
                 script_str.push_str("target remote :5039\n");
                 script_str.push_str(&format!("set solib-search-path \
@@ -508,11 +514,6 @@ actual:\n\
                     }
                 }
 
-                let tool_path = match self.config.android_cross_path.to_str() {
-                    Some(x) => x.to_owned(),
-                    None => self.fatal("cannot find android cross path")
-                };
-
                 let debugger_script = self.make_out_name("debugger.script");
                 // FIXME (#9639): This needs to handle non-utf8 paths
                 let debugger_opts =