deps: V8: fix debug build
This is a (very) partial cherry-pick of upstream change v8/v8@f8fddd6b13 that fixes the following debug mode build error with clang:
../deps/v8/src/codegen/register.h:29:48: error: member reference
base type 'RegListBase' is not a structure or union
int num_different_regs = RegListBase{regs...}.Count();