Support tailoring clang --target flag based on min sdk version.

Bug: 215748260
Test: Tested using aquery to verify the --target flag; add added unit

Change-Id: Ifb13509db7d1e110316aa44d55ec9cccaa4b83d1
diff --git a/bp2build/cc_library_conversion_test.go b/bp2build/cc_library_conversion_test.go
index 8fde655..d84e156 100644
--- a/bp2build/cc_library_conversion_test.go
+++ b/bp2build/cc_library_conversion_test.go
@@ -115,6 +115,8 @@
         },
     },
     include_build_directory: false,
+    sdk_version: "current",
+    min_sdk_version: "29",
 }
 `,
 		expectedBazelTargets: makeCcLibraryTargets("foo-lib", attrNameToString{
@@ -140,6 +142,8 @@
         "//build/bazel/platforms/os:linux_bionic": ["bionic.cpp"],
         "//conditions:default": [],
     })`,
+      "sdk_version": `"current"`,
+      "min_sdk_version": `"29"`,
 		}),
 	})
 }