Build with zig without gn and ninja.

[fubark]

Currently passing in -Dzig-toolchain injects "zig c++" into the compile/link commands but the overall build is still initiated with gn and ninja. It would be nice to build v8 directly with zig by just adding the necessary sources and flags. There are some prebuild steps that also need to be ported to build.zig like compiling torque files, running mksnapshot, and linking the resulting snapshot (with all the builtins) into the final v8 build.