BaseTools: Change CLANG8ELF to CLANGDWARF
    
    CLANGDWARF is more proper because it's similar to CLANGPDB that generates
    PE images but with DWARF debug symbols.
    This toolchain is needed for creating ELF format universal payload that
    follows https://universalpayload.github.io/documentation/.
    
    Signed-off-by: Ray Ni <ray.ni@intel.com>
    Cc: Bob Feng <bob.c.feng@intel.com>
    Cc: Liming Gao <gaoliming@byosoft.com.cn>
    Cc: Yuwei Chen <yuwei.chen@intel.com>
    Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>