Revisit allow(...)ed lints

[tgross35]

In particular, overflowing_literals would have caught #3584. We might be okay getting rid of a few of these

libc/src/lib.rs

Lines 5 to 18 in aff3132

	renamed_and_removed_lints, // Keep this order.
	unknown_lints, // Keep this order.
	bad_style,
	overflowing_literals,
	improper_ctypes,
	// This lint is renamed but we run CI for old stable rustc so should be here.
	redundant_semicolon,
	redundant_semicolons,
	unused_macros,
	unused_macro_rules,
	// FIXME: temporarily allow dead_code to fix CI:
	// - https://github.com/rust-lang/libc/issues/3740
	// - https://github.com/rust-lang/rust/pull/126456
	dead_code,

.

[tgross35]

On that note: we should make sure to separate flags intended for clone vs. anything that goes into clone_args for clone3, which uses 64-bit flags.