Fix LLVM ABI NAME for riscv64imac-unknown-nuttx-elf

This patch fix https://github.com/rust-lang/rust/issues/129825

For the riscv64imac target, the LLVM ABI NAME should be "lp64",
which is the default ABI if not specified for the riscv64imac target.

compiler/rustc_target/src/spec/targets/riscv64imac_unknown_nuttx_elf.rs

@@ -21,7 +21,6 @@ pub fn target() -> Target {
             os: "nuttx".into(),
             linker_flavor: LinkerFlavor::Gnu(Cc::No, Lld::Yes),
             linker: Some("rust-lld".into()),
-            llvm_abiname: "lp64d".into(),
             cpu: "generic-rv64".into(),
             max_atomic_width: Some(64),
             features: "+m,+a,+c".into(),