BaseTools: Change CLANG8ELF to CLANGDWARF

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>