* Allow FFI-unsafe warnings for u128/i128
Handle new warnings on nightly, and we shouldn't need to worry about
these with compiler-builtins since this is tied to a particular compiler.
* Clean up crate attributes
* No need for stability marker
* Rustdoc docs not used for this crate
* Remove old build-system related cruft from rustc itself.
* Run `cargo fmt`
As of LLVM 9.0, certain calls to memcmp may be converted to bcmp, which I guess
could save a single subtraction on some architectures. [1]
bcmp is just like memcmp except instead of returning the difference between the
two differing bytes, it returns non-zero instead. As such, memcmp is a valid
implementation of bcmp.
If we care about size, bcmp should just call memcmp.
If we care about speed, we can change bcmp to look like this instead:
```rust
pub unsafe extern "C" fn bcmp(s1: *const u8, s2: *const u8, n: usize) -> i32 {
let mut i = 0;
while i < n {
let a = *s1.offset(i as isize);
let b = *s2.offset(i as isize);
if a != b {
return 1;
}
i += 1;
}
0
}
```
In this PR I do not address any changes which may or may not be needed for arm
aebi as I lack proper test hardware.
[1]: https://releases.llvm.org/9.0.0/docs/ReleaseNotes.html#noteworthy-optimizations
This turns the following backtrace,
```
>> bt
#0 0x0000555555576f73 in __rust_probestack () at /cargo/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.14/src/probestack.rs:55
Backtrace stopped: Cannot access memory at address 0x7fffff7fedf0
```
To this:
```
>>> bt
#0 0x0000555555574e47 in __rust_probestack ()
#1 0x00005555555595ba in test::main ()
#2 0x00005555555594f3 in std::rt::lang_start::{{closure}} ()
#3 0x0000555555561ae3 in std::panicking::try::do_call ()
#4 0x000055555556595a in __rust_maybe_catch_panic ()
#5 0x000055555555af9b in std::rt::lang_start_internal ()
#6 0x00005555555594d5 in std::rt::lang_start ()
#7 0x000055555555977b in main ()
```
The -Zlower-128bit-ops feature is completely broken, as libcore needs
those lang items to compile with this feature, but they are only
provided by compiler_builtins, which itself depends on libcore.
According to rust-lang/rust#58969 the feature never got finished.
This commit removes the associated lang items and replaces them with
normal unmangled functions, when there is no existing intrinsic. This
makes it easier for alternative codegen backends to implement 128bit
integer support.
