TigerlakeSiliconPkg: Fix build error with GCC5

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3224



Fix build errors caused by "-Werror=unused-but-set-variable" with GCC5.
These build errors occur only with DEBUG target because
RELEASE_GCC5_X64_CC_FLAGS has "-Wno-unused-but-set-variable",
but DEBUG_GCC5_X64_CC_FLAGS doesn't.

Cc: Sai Chaganty <rangasai.v.chaganty@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Heng Luo <heng.luo@intel.com>
Signed-off-by: Takuto Naito <naitaku@gmail.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Heng Luo <heng.luo@intel.com>