rename --verbose to --verbose-spawn and limit the scope of what it does

[andrewrk]

Extracted from https://github.com/ziglang/zig/pull/14647.

Redefine it to: print to stderr the commandline before spawning any child process.

Audit all uses of the verbose flag and all child process invocations.

https://github.com/ziglang/zig/blob/68c7261e1daf7787dcbf0cd6f9b01e5678d20a93/lib/build_runner.zig#L117-L118

https://github.com/ziglang/zig/blob/68c7261e1daf7787dcbf0cd6f9b01e5678d20a93/lib/build_runner.zig#L946