[llvm-exegesis] Fix unused variable 'VAddressSpaceCeiling' warning (NFC)

/Users/jiefu/llvm-project/llvm/tools/llvm-exegesis/lib/X86/Target.cpp:52:33: error: unused variable 'VAddressSpaceCeiling' [-Werror,-Wunused-const-variable]
static constexpr const intptr_t VAddressSpaceCeiling = 0x0000800000000000;
                                ^
1 error generated.