perf build: Add clang and rust target flags for LoongArch

Add missing CLANG_TARGET_FLAGS_loongarch and RUST_TARGET_FLAGS_loongarch
so that perf can be built with clang and enable rust cross compilation.

Cc: stable@vger.kernel.org
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Dmitrii Dolgov <9erthalion6@gmail.com>
Signed-off-by: Haiyong Sun <sunhaiyong@loongson.cn>
Signed-off-by: WANG Rui <wangrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>