Express no{,_lib}crt via features vs attrs

Given that we can map them directly to disabling the
corresponding Bazel features: `-{link_,use_lib}crt`

Test: Existing (adapted) Unit Tests
Test: bp2build.sh
Change-Id: Ib502f6fb929ace8e86a1001e3cc21f399317500c
diff --git a/cc/linker.go b/cc/linker.go
index 371d78d..e49b97d 100644
--- a/cc/linker.go
+++ b/cc/linker.go
@@ -237,29 +237,14 @@
 	Exclude_shared_libs []string `android:"arch_variant"`
 }
 
-func invertBoolPtr(value *bool) *bool {
-	if value == nil {
-		return nil
-	}
-	ret := !(*value)
-	return &ret
+func (blp *BaseLinkerProperties) crt() bool {
+	// Since crt is enabled for almost every module compiling against the Bionic runtime,
+	// we interpret `nil` as  enabled.
+	return blp.Nocrt == nil || !*blp.Nocrt
 }
 
-func (blp *BaseLinkerProperties) crt() *bool {
-	val := invertBoolPtr(blp.Nocrt)
-	if val != nil && *val {
-		// == True
-		//
-		// Since crt is enabled for almost every module compiling against the Bionic runtime,
-		// use `nil` when it's enabled, and rely on the Starlark macro to set it to True by default.
-		// This keeps the BUILD files clean.
-		return nil
-	}
-	return val // can be False or nil
-}
-
-func (blp *BaseLinkerProperties) libCrt() *bool {
-	return invertBoolPtr(blp.No_libcrt)
+func (blp *BaseLinkerProperties) libCrt() bool {
+	return blp.No_libcrt == nil || !*blp.No_libcrt
 }
 
 func NewBaseLinker(sanitize *sanitize) *baseLinker {
@@ -392,7 +377,7 @@
 
 	if ctx.toolchain().Bionic() {
 		// libclang_rt.builtins has to be last on the command line
-		if !Bool(linker.Properties.No_libcrt) && !ctx.header() {
+		if linker.Properties.libCrt() && !ctx.header() {
 			deps.UnexportedStaticLibs = append(deps.UnexportedStaticLibs, config.BuiltinsRuntimeLibrary(ctx.toolchain()))
 		}
 
@@ -415,7 +400,7 @@
 			ctx.PropertyErrorf("system_shared_libs", "libdl must be after libc")
 		}
 	} else if ctx.toolchain().Musl() {
-		if !Bool(linker.Properties.No_libcrt) && !ctx.header() {
+		if linker.Properties.libCrt() && !ctx.header() {
 			deps.UnexportedStaticLibs = append(deps.UnexportedStaticLibs, config.BuiltinsRuntimeLibrary(ctx.toolchain()))
 		}
 	}