Add libstd and libcore Cargo features "panic_immediate_abort"

It stop asserts and panics from libstd to automatically
include string output and formatting code.

Use case: developing static executables smaller than 50 kilobytes,
where usual formatting code is excessive while keeping debuggability
in debug mode.

May resolve rust-lang#54981.

Author: vi

src/libstd/panicking.rs

@@ -355,9 +355,15 @@ pub fn rust_begin_panic(info: &PanicInfo) -> ! {
 #[unstable(feature = "libstd_sys_internals",
            reason = "used by the panic! macro",
            issue = "0")]
-#[inline(never)] #[cold]
+#[cold]
+#[cfg_attr(not(feature="panic_immediate_abort"),inline(never))]
+#[cfg_attr(    feature="panic_immediate_abort" ,inline)]
 pub fn begin_panic_fmt(msg: &fmt::Arguments,
                        file_line_col: &(&'static str, u32, u32)) -> ! {
+    if cfg!(feature = "panic_immediate_abort") {
+        unsafe { intrinsics::abort() }
+    };
+
     let (file, line, col) = *file_line_col;
     let info = PanicInfo::internal_constructor(
         Some(msg),
@@ -419,8 +425,15 @@ fn continue_panic_fmt(info: &PanicInfo) -> ! {
            reason = "used by the panic! macro",
            issue = "0")]
 #[cfg_attr(not(test), lang = "begin_panic")]
-#[inline(never)] #[cold] // avoid code bloat at the call sites as much as possible
+// avoid code bloat at the call sites as much as possible
+// inline(never) is required even in panic_immediate_abort mode, lest linker error
+#[inline(never)]
+#[cold]
 pub fn begin_panic<M: Any + Send>(msg: M, file_line_col: &(&'static str, u32, u32)) -> ! {
+    if cfg!(feature = "panic_immediate_abort") {
+        unsafe { intrinsics::abort() }
+    };
+
     // Note that this should be the only allocation performed in this code path.
     // Currently this means that panic!() on OOM will invoke this code path,
     // but then again we're not really ready for panic on OOM anyway. If