parallelizing more actions

[Efraim Flashner]

While rebuilding glibc-final on aarch64 I realized that the 'strip phase
took 235 seconds. The relevant code for 'strip from gnu-build-system is
in guix/build/gnu-build-system.scm, starting at line 340, with the
actual stripping starting at 398. When I changed 'for-each' to
'par-for-each' the time dropped from 235 seconds to 215, about an 8.5%
savings. I'm pretty sure most of that time was spent failing to strip
certain files, but it is still a savings.

I'm guessing there are other parts that can be parallelized in a similar
manner.

As for par-for-each (as many threads as cores on the machine) vs
n-par-for-each (n threads), I think it would be better to use
n-par-for-each, but I didn't quickly see a way to limit based on the
number of cores offered to each builder.

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