feat: add rust.rustflags option to bootstrap.toml

This makes easy to persistently pass any flag to the compiler when building rustc.
For example you can use a different linker by putting the following in
`bootstrap.toml`:
```toml
[rust]
rustflags = ["-Clinker=clang", "-Clink-arg=--ld-path=wild"]
```

Author: karolzwolak

src/bootstrap/src/core/builder/cargo.rs

@@ -601,6 +601,11 @@ impl Builder<'_> {
         }
 
         let mut rustflags = Rustflags::new(target);
+
+        for arg in &self.config.rust_rustflags {
+            rustflags.arg(arg);
+        }
+
         if build_compiler_stage != 0 {
             if let Ok(s) = env::var("CARGOFLAGS_NOT_BOOTSTRAP") {
                 cargo.args(s.split_whitespace());

src/bootstrap/src/core/config/config.rs

@@ -224,6 +224,7 @@ pub struct Config {
     pub rust_std_features: BTreeSet<String>,
     pub rust_break_on_ice: bool,
     pub rust_parallel_frontend_threads: Option<u32>,
+    pub rust_rustflags: Vec<String>,
 
     pub llvm_profile_use: Option<String>,
     pub llvm_profile_generate: bool,
@@ -571,6 +572,7 @@ impl Config {
             bootstrap_override_lld_legacy: rust_bootstrap_override_lld_legacy,
             std_features: rust_std_features,
             break_on_ice: rust_break_on_ice,
+            rustflags: rust_rustflags,
         } = toml.rust.unwrap_or_default();
 
         let Llvm {
@@ -1431,6 +1433,7 @@ impl Config {
             rust_randomize_layout: rust_randomize_layout.unwrap_or(false),
             rust_remap_debuginfo: rust_remap_debuginfo.unwrap_or(false),
             rust_rpath: rust_rpath.unwrap_or(true),
+            rust_rustflags: rust_rustflags.unwrap_or_default(),
             rust_stack_protector,
             rust_std_features: rust_std_features
                 .unwrap_or(BTreeSet::from([String::from("panic-unwind")])),

src/bootstrap/src/core/config/toml/rust.rs

@@ -33,6 +33,7 @@ define_config! {
         channel: Option<String> = "channel",
         musl_root: Option<String> = "musl-root",
         rpath: Option<bool> = "rpath",
+        rustflags: Option<Vec<String>> = "rustflags",
         strip: Option<bool> = "strip",
         frame_pointers: Option<bool> = "frame-pointers",
         stack_protector: Option<String> = "stack-protector",
@@ -373,6 +374,7 @@ pub fn check_incompatible_options_for_ci_rustc(
         parallel_frontend_threads: _,
         bootstrap_override_lld: _,
         bootstrap_override_lld_legacy: _,
+        rustflags: _,
     } = ci_rust_config;
 
     // There are two kinds of checks for CI rustc incompatible options: