Mark gcc as gcc instead of compiler in Unix CC toolchain

This makes it possible to detect gcc specifically via a `config_setting` for the `@bazel_tools//tools/cpp:compiler` flag with value `gcc`.

RELNOTES: The `@bazel_tools//tools/cpp:compiler` flag now has the value `gcc` if the configured compiler is detected to be gcc rather than the generic value `compiler`. A branch for `gcc` may have to be added to `select` statements that do not have a default case that handles gcc appropriately.

Fixes #12707

Closes #16297.

PiperOrigin-RevId: 475467838
Change-Id: I9fa98ddd3c7f29742357596280dfc98c939f0253

src/test/shell/bazel/cc_integration_test.sh

@@ -1708,4 +1708,51 @@ EOF
   expect_log "in pkg/library.cpp: ''"
 }
 
+function test_compiler_flag_gcc() {
+  # The default macOS toolchain always uses XCode's clang.
+  [ "$PLATFORM" != "darwin" ] || return 0
+  type -P gcc || return 0
+
+  cat > BUILD.bazel <<'EOF'
+config_setting(
+    name = "gcc_compiler",
+    flag_values = {"@bazel_tools//tools/cpp:compiler": "gcc"},
+)
+
+cc_binary(
+  name = "main",
+  srcs = select({":gcc_compiler": ["main.cc"]}),
+)
+EOF
+  cat > main.cc <<'EOF'
+int main() {}
+EOF
+
+  bazel build //:main --repo_env=CC=gcc || fail "Expected compiler flag to have value 'gcc'"
+}
+
+function test_compiler_flag_clang() {
+  # TODO: The default toolchain always uses XCode's clang, but sets the compiler name to the generic
+  #  "compiler".
+  [ "$PLATFORM" != "darwin" ] || return 0
+  type -P clang || return 0
+
+  cat > BUILD.bazel <<'EOF'
+config_setting(
+    name = "clang_compiler",
+    flag_values = {"@bazel_tools//tools/cpp:compiler": "clang"},
+)
+
+cc_binary(
+  name = "main",
+  srcs = select({":clang_compiler": ["main.cc"]}),
+)
+EOF
+  cat > main.cc <<'EOF'
+int main() {}
+EOF
+
+  bazel build //:main --repo_env=CC=clang || fail "Expected compiler flag to have value 'clang'"
+}
+
 run_suite "cc_integration_test"

tools/cpp/unix_cc_configure.bzl

@@ -279,6 +279,18 @@ def _coverage_flags(repository_ctx, darwin):
 def _is_clang(repository_ctx, cc):
     return "clang" in repository_ctx.execute([cc, "-v"]).stderr
 
+def _is_gcc(repository_ctx, cc):
+    # GCC's version output uses the basename of argv[0] as the program name:
+    # https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/gcc.cc;h=158461167951c1b9540322fb19be6a89d6da07fc;hb=HEAD#l8728
+    return repository_ctx.execute([cc, "--version"]).stdout.startswith("gcc ")
+
+def _get_compiler_name(repository_ctx, cc):
+    if _is_clang(repository_ctx, cc):
+        return "clang"
+    if _is_gcc(repository_ctx, cc):
+        return "gcc"
+    return "compiler"
+
 def _find_generic(repository_ctx, name, env_name, overriden_tools, warn = False, silent = False):
     """Find a generic C++ toolchain tool. Doesn't %-escape the result."""
 
@@ -492,7 +504,7 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
             "%{compiler}": escape_string(get_env_var(
                 repository_ctx,
                 "BAZEL_COMPILER",
-                "clang" if is_clang else "compiler",
+                _get_compiler_name(repository_ctx, cc),
                 False,
             )),
             "%{abi_version}": escape_string(get_env_var(