BaseTools: Add new CLANG8ELF tool chain for new LLVM/CLANG8

BaseTools: Add new CLANG8ELF tool chain for new LLVM/CLANG8

BZ:https://bugzilla.tianocore.org/show_bug.cgi?id=1603
LLVM/CLANG8 formal release http://releases.llvm.org/download.html#8.0.0
It can be downloaded and installed in Windows/Linux/Mac OS.
CLANG8ELF tool chain is added to generate ELF image, and convert to PE/COFF.
On Windows OS, set CLANG_HOST_BIN=n, set CLANG8_BIN=LLVM installed directory
For example:
  set CLANG_HOST_BIN=n # use windows nmake
  set CLANG8_BIN=C:\Program Files\LLVM\bin\
On Linux/Mac, set CLANG8_BIN=LLVM installed directory

This tool chain can be used to compile the firmware code. On windows OS,
Visual Studio is still required to compile BaseTools C tools and nmake.exe.
On Linux/Mac OS, gcc is used to compile BaseTools C tools. make is used
for makefile.

This tool chain is verified on OVMF Ia32, X64 and Ia32X64 to boot Shell.
This tool chain is verified in Windows/Linux and Mac OS.

Signed-off-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Feng Bob C <bob.c.feng@intel.com>