Fix backtraces on i686-pc-windows-gnu by disabling FPO

This might have performance implications. But do note that MSVC
disables FPO by default nowadays and it's use is limited in exception
heavy languages like C++.

Closes: rust-lang#28218

src/librustc_back/target/i686_pc_windows_gnu.rs

@@ -14,6 +14,7 @@ pub fn target() -> TargetResult {
     let mut base = super::windows_base::opts();
     base.cpu = "pentium4".to_string();
     base.max_atomic_width = Some(64);
+    base.eliminate_frame_pointer = false; // Required for backtraces
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address
     // space available to x86 Windows binaries on x86_64.

src/test/run-pass/backtrace-debuginfo.rs

@@ -37,7 +37,6 @@ macro_rules! dump_and_die {
                     target_os = "ios",
                     target_os = "android",
                     all(target_os = "linux", target_arch = "arm"),
-                    all(target_os = "windows", target_pointer_width = "32"),
                     target_os = "freebsd",
                     target_os = "dragonfly",
                     target_os = "bitrig",

src/test/run-pass/backtrace.rs

@@ -104,10 +104,6 @@ fn runtest(me: &str) {
 }
 
 fn main() {
-    if cfg!(windows) && cfg!(target_env = "gnu") && cfg!(target_pointer_width = "32") {
-        return
-    }
-
     let args: Vec<String> = env::args().collect();
     if args.len() >= 2 && args[1] == "fail" {
         foo();