[DO NOT MERGE] update target feature following LLVM API change

LLVM commit llvm/llvm-project@e817966
renamed the `unaligned-scalar-mem` target feature to `fast-unaligned-access`.

Author: krasimirgg

tests/ui/abi/riscv-discoverability-guidance.rs

@@ -2,9 +2,9 @@
 // revisions: riscv32 riscv64
 //
 // [riscv32] needs-llvm-components: riscv
-// [riscv32] compile-flags: --target=riscv32i-unknown-none-elf -C target-feature=-unaligned-scalar-mem --crate-type=rlib
+// [riscv32] compile-flags: --target=riscv32i-unknown-none-elf -C target-feature=-fast-unaligned-access --crate-type=rlib
 // [riscv64] needs-llvm-components: riscv
-// [riscv64] compile-flags: --target=riscv64gc-unknown-none-elf -C target-feature=-unaligned-scalar-mem --crate-type=rlib
+// [riscv64] compile-flags: --target=riscv64gc-unknown-none-elf -C target-feature=-fast-unaligned-access --crate-type=rlib
 #![no_core]
 #![feature(
     no_core,