Revert "Prevent clang from using the x18 register on arm64."

This reverts commit 98d8580c3a.

Change no longer needed with the new LLVM toolchain update.

Bug: 112907825
Bug: 111759196
Change-Id: Ife3ce47a602dd850ac84bb51abc70c0ca157776e

cc/lto.go

@@ -113,13 +113,6 @@ func (lto *lto) flags(ctx BaseModuleContext, flags Flags) Flags {
 			flags.LdFlags = append(flags.LdFlags, "-Wl,-plugin-opt,-inline-threshold=0")
 			flags.LdFlags = append(flags.LdFlags, "-Wl,-plugin-opt,-unroll-threshold=0")
 		}
-
-		if ctx.Arch().ArchType == android.Arm64 {
-			// Prevent use of x18 register on arm64.
-			// TODO(pcc): Remove this flag once we upgrade past LLVM r340889
-			// which does this by default on Android.
-			flags.LdFlags = append(flags.LdFlags, "-Wl,-plugin-opt,-mattr=+reserve-x18")
-		}
 	}
 	return flags
 }