Rebuild V with the passed options.

Usage:
  v self
  v self -prod
  v self -b fastc
  v -b arm64 self
  v -b arm64 self x2

Options:
  All other options are passed to the build command. (e.g. -prod)
  Plain self-builds compile the full `cmd/v` CLI, which embeds and defaults to V3 on macOS,
  Linux, and BSD.
  Plain self-builds keep the FastC backend, so the new compiler accepts `-b fastc`; pass
  `-d skip_fastc` to leave it out.
  On Apple Silicon, regular self-builds use bundled TCC by default; `CC` or `-cc` overrides it.
  -prod uses a single build of one C unit with LTO instead of a PGO cycle. That build runs
  the C compiler on one thread; add -parallel-cc for a faster build of a slower compiler.
  On macOS and Linux, `-b fastc` builds the standalone experimental V3 FastC compiler.
  `-prod` cannot be combined with `-b fastc`.
  FastC replacement builds (`xN` without `-o`) reject options that the standalone compiler
  cannot preserve across every generation.
  On macOS ARM64, `-b arm64` builds the full CLI with the native ARM64 backend.
  First run `v -compile-backend arm64 self` if the current compiler lacks that backend.
  Native builds retain the C backend and leave out FastC by default.
  `xN` repeats replacement builds, using the newly built compiler for the next build.
  Add `-o v_arm64` to keep the current compiler and write a separate executable.
  Native self-builds do not add C compiler or preallocation options, and report native
  compilation failures directly without retrying through the C bootstrap.
