Using LLVM compiler set to build BaseTools in Linux

Using LLVM compiler set to build BaseTools in Linux

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2842

To use LLVM to build BaseTools, first set the CLANG_BIN environment value,
and add "CXX=llvm" to choose LLVM compiler set when using make command.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Yuwei Chen<yuwei.chen@intel.com>