BaseTools-CLANG38: Add -O3 in DLINK2 flag

CLANG38 build fail after CC_FLAG is added in the link rule.
This failure is because the CLANG38 enable the LTO through LLVMgold.so
linker plugin, but the LLVMgold.so plugin cannot accept the clang -Oz
CC flag as build option. After CC_FLAG is added in the link rule,
the LLVMgold.so plugin reports linking error. LLVMgold.so only accept
-O0 ~ -O3, and you can see it in the LLVM gold plugin source code
in below:

http://llvm.org/svn/llvm-project/llvm/tags/RELEASE_380/final/tools/gold/
gold-plugin.cpp line173:

if (opt[1] < '0' || opt[1] > '3')
   message(LDPL_FATAL, "Optimization level must be between 0 and 3");

Add -O3 in the *_CLANG38_*_DLINK2_FLAGS to override the -Oz flag in
*_CLANG38_*_CC_FLAGS to pass LLVMgold.so linking.

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

BaseTools/Conf/tools_def.template

@@ -5489,7 +5489,7 @@ DEFINE CLANG38_ALL_CC_FLAGS         = DEF(GCC44_ALL_CC_FLAGS) -Wno-empty-body -f
 DEBUG_CLANG38_IA32_CC_FLAGS         = DEF(CLANG38_ALL_CC_FLAGS) -m32 -Oz -flto -march=i586 DEF(CLANG38_IA32_TARGET) -g
 RELEASE_CLANG38_IA32_CC_FLAGS       = DEF(CLANG38_ALL_CC_FLAGS) -m32 -Oz -flto -march=i586 DEF(CLANG38_IA32_TARGET)
 *_CLANG38_IA32_DLINK_FLAGS          = DEF(GCC5_IA32_X64_DLINK_FLAGS) -Wl,-Oz -Wl,-melf_i386 -Wl,--oformat=elf32-i386
-*_CLANG38_IA32_DLINK2_FLAGS         = DEF(GCC5_IA32_DLINK2_FLAGS)
+*_CLANG38_IA32_DLINK2_FLAGS         = DEF(GCC5_IA32_DLINK2_FLAGS) -O3
 *_CLANG38_IA32_RC_FLAGS             = DEF(GCC_IA32_RC_FLAGS)
 *_CLANG38_IA32_OBJCOPY_FLAGS        =
 *_CLANG38_IA32_NASM_FLAGS           = -f elf32
@@ -5518,7 +5518,7 @@ RELEASE_CLANG38_IA32_CC_FLAGS       = DEF(CLANG38_ALL_CC_FLAGS) -m32 -Oz -flto -
 DEBUG_CLANG38_X64_CC_FLAGS         = DEF(CLANG38_ALL_CC_FLAGS) -m64 "-DEFIAPI=__attribute__((ms_abi))" -mno-red-zone -mcmodel=small -fpie -Oz -flto DEF(CLANG38_X64_TARGET) -g
 RELEASE_CLANG38_X64_CC_FLAGS       = DEF(CLANG38_ALL_CC_FLAGS) -m64 "-DEFIAPI=__attribute__((ms_abi))" -mno-red-zone -mcmodel=small -fpie -Oz -flto DEF(CLANG38_X64_TARGET)
 *_CLANG38_X64_DLINK_FLAGS          = DEF(GCC5_IA32_X64_DLINK_FLAGS) -Wl,-Oz -Wl,-melf_x86_64 -Wl,--oformat=elf64-x86-64 -Wl,-pie -mcmodel=small
-*_CLANG38_X64_DLINK2_FLAGS         = DEF(GCC5_X64_DLINK2_FLAGS)
+*_CLANG38_X64_DLINK2_FLAGS         = DEF(GCC5_X64_DLINK2_FLAGS) -O3
 *_CLANG38_X64_RC_FLAGS             = DEF(GCC_X64_RC_FLAGS)
 *_CLANG38_X64_OBJCOPY_FLAGS        =
 *_CLANG38_X64_NASM_FLAGS           = -f elf64