For builtins, only build the builtins target

It seemed like a nice idea to specify the scope via cmake and then
just use the generic `install` target but problems like f546294 (Disable
ctx_profile to fix clangrt-builtins, 2024-11-27) happen.  Recently, there's
some similar but yet-unidentified problem causing similar symptoms (attempts
to build sanitizer code in compiler-rt that's not yet supported, e.g.).

Instead, let's just build the stuff we need by specifying the right target.

Signed-off-by: Brian Cain <brian.cain@oss.qualcomm.com>

build-toolchain.sh

@@ -120,7 +120,7 @@ build_clang_rt_builtins() {
 		-B ./obj_clang_rt \
 		-S ./llvm-project/compiler-rt
 
-	cmake --build ./obj_clang_rt -- -v install
+	cmake --build ./obj_clang_rt -- -v install-builtins
 }