Allow sparse GCC version support per architecture

Right now we assume that every GCC version is available for every
architecture. E.g. we always try to use the GCC 4.8 for auditing.
For Darwin Aarch64, we won't have any GCC shard smaller than 11.
We do already support falling back to the closest available version
if the preferred version is not supported. This simply extends
that check to only include compiler shards that we actually have
artifacts for.

src/Rootfs.jl

@@ -476,11 +476,19 @@ function choose_shards(p::Platform;
             preferred_llvm_version::VersionNumber = getversion(LLVM_builds[end]),
         )
 
-    GCC_build = select_gcc_version(p, GCC_builds, preferred_gcc_version)
-    LLVM_build = select_closest_version(preferred_llvm_version, getversion.(LLVM_builds))
     # Our host platform is x86_64-linux-musl
     host_platform = Linux(:x86_64; libc=:musl)
 
+    make_gcc_shard(GCC_build, target) = CompilerShard("GCCBootstrap", GCC_build, host_platform, archive_type; target=target)
+
+    this_platform_GCC_builds = filter(GCC_builds) do GCC_build
+        make_gcc_shard(getversion(GCC_build), p) in all_compiler_shards() &&
+        make_gcc_shard(getversion(GCC_build), host_platform) in all_compiler_shards()
+    end
+
+    GCC_build = select_gcc_version(p, this_platform_GCC_builds, preferred_gcc_version)
+    LLVM_build = select_closest_version(preferred_llvm_version, getversion.(LLVM_builds))
+
     shards = CompilerShard[]
     if isempty(bootstrap_list)
         append!(shards, [
@@ -492,14 +500,14 @@ function choose_shards(p::Platform;
         platform_match(a, b) = ((typeof(a) <: typeof(b)) && (arch(a) == arch(b)) && (libc(a) == libc(b)))
         if :c in compilers
             append!(shards, [
-                CompilerShard("GCCBootstrap", GCC_build, host_platform, archive_type; target=p),
+                make_gcc_shard(GCC_build, p),
                 CompilerShard("LLVMBootstrap", LLVM_build, host_platform, archive_type),
             ])
             # If we're not building for the host platform, then add host shard for host tools
             if !platform_match(p, host_platform)
                 append!(shards, [
                     CompilerShard("PlatformSupport", ps_build, host_platform, archive_type; target=host_platform),
-                    CompilerShard("GCCBootstrap", GCC_build, host_platform, archive_type; target=host_platform),
+                    make_gcc_shard(GCC_build, host_platform)
                 ])
             end
         end
@@ -517,7 +525,7 @@ function choose_shards(p::Platform;
 
                 # We have to add these as well for access to linkers and whatnot for Rust.  Sigh.
                 push!(shards, CompilerShard("PlatformSupport", ps_build, host_platform, archive_type; target=Rust_host))
-                push!(shards, CompilerShard("GCCBootstrap", GCC_build, host_platform, archive_type; target=Rust_host))
+                push!(shards, make_gcc_shard(GCC_build, Rust_host))
             end
             if !platform_match(p, host_platform)
                 push!(shards, CompilerShard("RustToolchain", Rust_build, Rust_host, archive_type; target=host_platform))
@@ -532,6 +540,7 @@ function choose_shards(p::Platform;
             versions = [cs.version for cs in all_compiler_shards()
                 if cs.name == name && cs.archive_type == archive_type && (something(cs.target, p) == p)
             ]
+            isempty(versions) && error("No latest shard found for $name")
             return maximum(versions)
         end