Handle gdb command failure gracefully in compiletest

Previously, if the gdb command was available, but threw an error, compiletest would panic.  This is obviously not good. Now, gdb is treated as missing if calling `gdb --version` does not output anything on stdout.

src/tools/compiletest/src/main.rs

@@ -736,17 +736,12 @@ fn analyze_gdb(gdb: Option<String>) -> (Option<String>, Option<u32>, bool) {
         Some(ref s) => s,
     };
 
-    let version_line = Command::new(gdb)
-        .arg("--version")
-        .output()
-        .map(|output| {
-            String::from_utf8_lossy(&output.stdout)
-                .lines()
-                .next()
-                .unwrap()
-                .to_string()
-        })
-        .ok();
+    let mut version_line = None;
+    if let Ok(output) = Command::new(gdb).arg("--version").output() {
+        if let Some(first_line) = String::from_utf8_lossy(&output.stdout).lines().next() {
+            version_line = Some(first_line.to_string());
+        }
+    }
 
     let version = match version_line {
         Some(line) => extract_gdb_version(&line),