Makefile: Dont override NM and OBJCOPY if LLVM=1

This workaround was introduced in [1] to remedy relr build when using gnu binutils. But due to [2] LLVMNM and LLVMOBJCOPY wasn't being set during LLVM=1 build.
So when CONFIG_RELR is enabled along side with LLVM=1, NM and OBJCOPY get overriden by empty variables and then get exported. Thus override them only when compiling without LLVM=1.

[1]: https://github.com/dakkshesh07/parallax_kernel_realme_RMX1921/commit/5a785da6d1845cf4653952d285b5987d521741ac
[2]: https://github.com/dakkshesh07/parallax_kernel_realme_RMX1921/commit/b2c7ab3ce7052cf4472a4cdd5154712dca0e0535


Signed-off-by: Dakkshesh <dakkshesh5@gmail.com>