fixed zstd-pgo target for GCC

fixed zstd-pgo target for GCC

Since your Makefile uses obj/$(HASH_DIR) for object files, this code does not work correctly for GCC. Because profiles are saved in one directory, and are expected in another when reading.

`$(RM) zstd *.o` - this line doesn't delete object files.

Clang stores profiles in the current directory, so the problem doesn't appear when compiling with Clang.

Also this code will work if BUILD_DIR is set.