Add .insn assembly directive.

`.insn` was added to gnu-as in 2018 in
bminor/binutils-gdb@0e35537
and in llvm in llvm-14 in
llvm/llvm-project@2838797
(I think, from some quick googling).

Author: CyoeeA9e

src/inline-assembly.md

@@ -570,6 +570,7 @@ The following directives are guaranteed to be supported by the assembler:
 - `.global`
 - `.globl`
 - `.inst`
+- `.insn`
 - `.lcomm`
 - `.long`
 - `.octa`