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.
  On non-Windows hosts, -prod uses PGO with Clang when llvm-profdata is available.
  It builds an instrumented compiler, trains it on uncached self-compilation, then builds
  the optimized compiler. This takes longer to build the production compiler.
  Other C toolchains and failed PGO attempts use a regular production build.
  Add -parallel-cc to skip PGO and compile the generated C units in parallel.
  Explicit -no-prod-options or profile-related -cflags also skip automatic PGO.
  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.
