BaseTools-Conf:Introduce CLANG38 new toolchain for x86

BaseTools-Conf:Introduce CLANG38 new toolchain for x86

This adds support for LLVM 3.8.x in LTO mode for IA32 and X64.
CLANG38 enable LLVM Link Time Optimization (LTO) and code size
optimization flag (-Oz) by default for aggressive code size
improvement. CLANG38 X64 code is small code model + PIE.

CLANG LTO needs PIE in link flags to generate PIE code correctly,
otherwise the PIE is not really enabled. (e.g. OvmfPkgX64 will
hang in 64bits SEC at high address because of small model code
displacement overflow).

Test pass platforms: OVMF (OvmfPkgIa32.dsc, OvmfPkgX64.dsc and
OvmfPkgIa32X64.dsc).
Test compiler and linker version: LLVM 3.8, GNU ld 2.26.

Example steps to use the CLANG38 tool chain to build OVMF platform:
1. Download and extract the llvm 3.8.0 Pre-Built Binaries from
http://www.llvm.org/releases/ (e.g. http://www.llvm.org/releases/
3.8.0/clang+llvm-3.8.0-x86_64-linux-gnu-ubuntu-16.04.tar.xz and
extract it as ~/clang38).
2. Copy LLVMgold.so from https://github.com/shijunjing/edk2/blob/
llvm/BaseTools/Bin/LLVMgold.so to above clang lib folder (e.g.
~/clang38/lib/LLVMgold.so)
3. Install new version linker with plugin support (e.g. ld 2.26 in
GNU Binutils 2.26 or Ubuntu16.04)
$ cd edk2
$ git checkout llvm
$ export CLANG38_BIN=path/to/your/clang38/
(e.g. export CLANG38_BIN=~/clang38/bin/)
$ source edksetup.sh
$ make -C BaseTools/Source/C
$ build -t CLANG38 -a X64 -p OvmfPkg/OvmfPkgX64.dsc -n 5 -b DEBUG
-DDEBUG_ON_SERIAL_PORT
$ cd edk2/Build/OvmfX64/DEBUG_CLANG38/FV
$ qemu-system-x86_64.exe -bios OVMF.fd -serial file:serial.log -m 4096
 -hda fat:.

If you want, you can build and install GNU Binutils 2.26 as below steps
in Ubuntu:
Download binutils-2.26 source code from http://ftp.gnu.org/gnu/binutils/
 and extract it to ~/binutils-2.26
$sudo apt-get install bison
$sudo apt-get install flex
Install other necessary binutils build tools if missing
$ mkdir build
$ cd build
$ ../binutils-2.26/configure --enable-gold --enable-plugins
--disable-werror --prefix=/usr
$ make -j 5
$ sudo make install

If you want, you can build LLVMgold.so as below steps
Download llvm-3.8.0 source code from http://www.llvm.org/releases/
3.8.0/llvm-3.8.0.src.tar.xz and extract it to ~/llvm-3.8.0.src
Download clang3.8.0 source code from http://www.llvm.org/releases/
3.8.0/cfe-3.8.0.src.tar.xz and extract it to ~/llvm-3.8.0.src/tools/clang
Refer http://clang.llvm.org/get_started.html to Install other necessary
clang build tools if missing
$ mkdir llvm38build
$ cd llvm38build
If your GNU Binutils 2.26 is in /home/jshi19/binutils-2.26,
$ cmake ../llvm-3.8.0.src -G "Unix Makefiles" -DCMAKE_BUILD_TYPE="Release"
-DLLVM_TARGETS_TO_BUILD="X86" -DCMAKE_VERBOSE_MAKEFILE=ON
-DCMAKE_CXX_COMPILER="/usr/bin/g++" -DCMAKE_C_COMPILER="/usr/bin/gcc"
-DLLVM_BINUTILS_INCDIR=/home/jshi19/binutils-2.26/include
$ make -j 5 LLVMgold The LLVMgold.so is in ~/llvm38build/lib/LLVMgold.so

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Steven Shi <steven.shi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>