Add riscv64a23-unknown-linux-gnu to build-manifest TARGETS

This enables rustup distribution of the rust-std component for the
riscv64a23-unknown-linux-gnu target, allowing users to install it via
`rustup target add riscv64a23-unknown-linux-gnu`.

Author: ZhongyaoChen

src/tools/build-manifest/src/main.rs

@@ -153,6 +153,7 @@ static TARGETS: &[&str] = &[
     "riscv32imafc-unknown-none-elf",
     "riscv32gc-unknown-linux-gnu",
     "riscv64imac-unknown-none-elf",
+    "riscv64a23-unknown-linux-gnu",
     "riscv64gc-unknown-hermit",
     "riscv64gc-unknown-none-elf",
     "riscv64gc-unknown-linux-gnu",