Add gcc bootstrap config section

Author: Kobzol

Diff for: config.example.toml

@@ -163,6 +163,11 @@
 # Custom CMake defines to set when building LLVM.
 #build-config = {}
 
+# =============================================================================
+# Tweaking how GCC is compiled
+# =============================================================================
+[gcc]
+
 # =============================================================================
 # General build configuration options
 # =============================================================================

Diff for: src/bootstrap/src/core/config/config.rs

@@ -676,6 +676,7 @@ pub(crate) struct TomlConfig {
     build: Option<Build>,
     install: Option<Install>,
     llvm: Option<Llvm>,
+    gcc: Option<Gcc>,
     rust: Option<Rust>,
     target: Option<HashMap<String, TomlTarget>>,
     dist: Option<Dist>,
@@ -710,7 +711,7 @@ trait Merge {
 impl Merge for TomlConfig {
     fn merge(
         &mut self,
-        TomlConfig { build, install, llvm, rust, dist, target, profile, change_id }: Self,
+        TomlConfig { build, install, llvm, gcc, rust, dist, target, profile, change_id }: Self,
         replace: ReplaceOpt,
     ) {
         fn do_merge<T: Merge>(x: &mut Option<T>, y: Option<T>, replace: ReplaceOpt) {
@@ -729,6 +730,7 @@ impl Merge for TomlConfig {
         do_merge(&mut self.build, build, replace);
         do_merge(&mut self.install, install, replace);
         do_merge(&mut self.llvm, llvm, replace);
+        do_merge(&mut self.gcc, gcc, replace);
         do_merge(&mut self.rust, rust, replace);
         do_merge(&mut self.dist, dist, replace);
 
@@ -995,6 +997,11 @@ define_config! {
     }
 }
 
+define_config! {
+    /// TOML representation of how the GCC build is configured.
+    struct Gcc {}
+}
+
 define_config! {
     struct Dist {
         sign_folder: Option<String> = "sign-folder",