Rollup merge of #133463 - taiki-e:aarch64-asm-x18, r=Amanieu

Fix handling of x18 in AArch64 inline assembly on ohos/trusty or with -Zfixed-x18

Currently AArch64 inline assembly allows using x18 on ohos/trusty or with -Zfixed-x18.

7db7489f9b/compiler/rustc_target/src/asm/aarch64.rs (L74-L76)

However, x18 is reserved in these environments and should not be allowed in the input/output operands of inline assemblies as it is in Android, Windows, etc..

7db7489f9b/compiler/rustc_target/src/spec/targets/aarch64_unknown_linux_ohos.rs (L19)
7db7489f9b/compiler/rustc_target/src/spec/targets/aarch64_unknown_trusty.rs (L18)
7db7489f9b/compiler/rustc_codegen_llvm/src/llvm_util.rs (L764-L771)

(As for ohos, +reserve-x18 is [redundant](c417b7a695 (diff-0ddf23e0bf2b28b2d05f842f087d1e6f694e8e06d1765e8d0f10d47fddcdff9c)) since 7a966b9188 that starting using llvm's ohos targets. So removed it from target-spec.)

This fix may potentially break the code for tier 2 target (aarch64-unknown-linux-ohos). (As for others, aarch64-unknown-trusty is tier 3 and -Zfixed-x18 is unstable so breaking them should be fine.)
However, in any case, it seems suspicious that the code that is broken by this was sound.

r? `@Amanieu`

`@rustbot` label O-AArch64 +A-inline-assembly

compiler/rustc_span/src/symbol.rs

@@ -1611,6 +1611,7 @@ symbols! {
         repr_simd,
         repr_transparent,
         require,
+        reserve_x18: "reserve-x18",
         residual,
         result,
         result_ffi_guarantees,