Remove the out/soong/.bootstrap directory.

It's better not to have state hidden in dotfiles (or rather,
dotdirectories) if we can help.

It's questionable whether the "linux-x86" path segment makes sense since
soong_build only ever runs on one operating system, but I didn't want to
rock the boat now.

Drive-by fixed some quoting fixes in rbc-run. Notably, I didn't wrap
`$@` into double quotes because I don't know whether the lack of double
quotes was intended or not.

Also drive-by fixed the fact that "out/soong" was added twice to the
directory name of bpglob. This turned out not to be a problem because
bpglob doesn't need to be explicitly built: if it's needed, it's
declared as an input of the glob files so it'll be built automatically
(at the cost of the first "null build" not actually being a null build)

Test: Presubmits.
Change-Id: I710d8d16cd8212059a0ca1ee95378505303eed83