ninja: Only use response files when needed

Writing rsp files on Windows is moderately expensive, so only use them
when the command line is long enough to need them.

This also makes the output of 'ninja -v' useful more often (something
like 'cl @exec@exe/main.c.obj.rsp' is not very useful if you don't have
the response file to look at)

For a rule where using a rspfile is possible, write rspfile and
non-rspfile versions of that rule.  Choose which one to use for each
build statement, depending on the anticpated length of the command line.

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