Rollup merge of #66661 - msizanoen1:riscv-gnu, r=alexcrichton

Add riscv64gc-unknown-linux-gnu target

This PR add the target, but doesn't build std on CI yet.
I have a port for `libc` crate and std which I will upstream soon after this target is added.

r? @alexcrichton

src/librustc_target/spec/mod.rs

@@ -487,6 +487,7 @@ fn $module() {
     ("riscv32imac-unknown-none-elf", riscv32imac_unknown_none_elf),
     ("riscv64imac-unknown-none-elf", riscv64imac_unknown_none_elf),
     ("riscv64gc-unknown-none-elf", riscv64gc_unknown_none_elf),
+    ("riscv64gc-unknown-linux-gnu", riscv64gc_unknown_linux_gnu),
 
     ("aarch64-unknown-none", aarch64_unknown_none),
     ("aarch64-unknown-none-softfloat", aarch64_unknown_none_softfloat),

src/librustc_target/spec/riscv64gc_unknown_linux_gnu.rs

@@ -0,0 +1,25 @@
+use crate::spec::{LinkerFlavor, Target, TargetOptions, TargetResult};
+
+pub fn target() -> TargetResult {
+    Ok(Target {
+        llvm_target: "riscv64-unknown-linux-gnu".to_string(),
+        target_endian: "little".to_string(),
+        target_pointer_width: "64".to_string(),
+        target_c_int_width: "32".to_string(),
+        target_env: "gnu".to_string(),
+        data_layout: "e-m:e-p:64:64-i64:64-i128:128-n64-S128".to_string(),
+        arch: "riscv64".to_string(),
+        target_os: "linux".to_string(),
+        target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
+        options: TargetOptions {
+            abi_blacklist: super::riscv_base::abi_blacklist(),
+            code_model: Some("medium".to_string()),
+            cpu: "generic-rv64".to_string(),
+            features: "+m,+a,+f,+d,+c".to_string(),
+            llvm_abiname: "lp64d".to_string(),
+            max_atomic_width: Some(64),
+            ..super::linux_base::opts()
+        },
+    })
+}