acle/hints: __dbg requires 'v7'

addresses #557 (comment)

Author: japaric

crates/core_arch/src/acle/hints.rs

@@ -72,8 +72,11 @@ pub unsafe fn __yield() {
 /// for the effect (if any) of this instruction and the meaning of the
 /// argument. This is available only when compliling for AArch32.
 // Section 10.1 of ACLE says that the supported arches are: 7, 7-M
-// LLVM says "instruction requires: thumb2" OR "instruction requires: armv7"
-#[cfg(target_feature = "v6t2")]
+// "The DBG hint instruction is added in ARMv7. It is UNDEFINED in the ARMv6 base architecture, and
+// executes as a NOP instruction in ARMv6K and ARMv6T2." - ARM Architecture Reference Manual ARMv7-A
+// and ARMv7-R edition (ARM DDI 0406C.c) sections D12.4.1 "ARM instruction set support" and D12.4.2
+// "Thumb instruction set support"
+#[cfg(target_feature = "v7")]
 #[inline(always)]
 #[rustc_args_required_const(0)]
 pub unsafe fn __dbg(imm4: u32) {