No longer build with external llvm < 14 on riscv

[slanterns]

rust-lang/stdarch#1271 includes the use of .insn and goes into Rust 1.59 in #92574, which is introduced in rust-lang/llvm-project#121 and #91528. However this makes rustc no longer work with external llvm < 14 on riscv platforms. Is this considered as a regression?

[slanterns]

Seems duplicate with rust-lang/stdarch#1291.