Remove obscure & rarely used ARM intrinsics

In almost all cases it is preferable to use the stable `asm!` instead of
calling these intrinsics.

This PR removes the following unstable intrinsics:
- `__breakpoint` Clang extension, not part of ACLE.
- `brk`: undocumented
- `_rev*`, `_clz*`, `_rbit*`: use methods on integer types instead
- `__ldrex`, `__strex`: deprecated in ACLE, hard to use correctly
- Register access API: API doesn't match ACLE, better to just use asm

Also considered for deletion, but not included in this PR:
- Barriers: `__isb`, `__dsb`, `__dmb`
- Hints: `__wfi`, `__wfe`, `__sev`, `__sevl`, `__yield`, `__nop`

Author: Amanieu

crates/core_arch/src/aarch64/armclang.rs

@@ -6,9 +6,6 @@
 //! [arm_ref]: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0073a/IHI0073A_arm_neon_intrinsics_ref.pdf
 //! [arm_dat]: https://developer.arm.com/technologies/neon/intrinsics
 
-mod v8;
-pub use self::v8::*;
-
 mod neon;
 pub use self::neon::*;
 
@@ -23,19 +20,8 @@ pub use self::prefetch::*;
 
 pub use super::arm_shared::*;
 
-mod armclang;
-
-pub use self::armclang::*;
-
 #[cfg(test)]
 use stdarch_test::assert_instr;
 
-/// Generates the trap instruction `BRK 1`
-#[cfg_attr(test, assert_instr(brk))]
-#[inline]
-pub unsafe fn brk() -> ! {
-    crate::intrinsics::abort()
-}
-
 #[cfg(test)]
 pub(crate) mod test_support;

crates/core_arch/src/aarch64/mod.rs

@@ -6,12 +6,6 @@
 //! [arm_ref]: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0073a/IHI0073A_arm_neon_intrinsics_ref.pdf
 //! [arm_dat]: https://developer.arm.com/technologies/neon/intrinsics
 
-mod armclang;
-pub use self::armclang::*;
-
-mod v6;
-pub use self::v6::*;
-
 // Supported arches: 6, 7-M. See Section 10.1 of ACLE (e.g. SSAT)
 #[cfg(any(target_feature = "v6", doc))]
 mod sat;
@@ -62,14 +56,6 @@ mod simd32;
 ))]
 pub use self::simd32::*;
 
-#[cfg(any(target_feature = "v7", doc))]
-mod v7;
-#[cfg(any(target_feature = "v7", doc))]
-pub use self::v7::*;
-
-mod ex;
-pub use self::ex::*;
-
 pub use crate::core_arch::arm_shared::*;
 
 #[cfg(test)]