uninative: Make patchelf modified files sparse

When we switched to recipe specific sysroots (rss), performance took a nose dive. Its
easy to blame rss but it turns out not to be entirely at fault.

Three configurations are compared here:

a) Pre-RSS (revision 45df694a9f472ac2f684aadac4d864c3dfdc48a7)
b) Post-RSS (revision 226a508da955439b881b2f0a544a3aee76e59919)
c) as b) with this change

Overall build times:

a) 22794.25user 2687.88system 30:32.84elapsed 1390%CPU (0avgtext+0avgdata 919056maxresident)k
b) 22677.25user 3238.79system 36:16.68elapsed 1190%CPU (0avgtext+0avgdata 918896maxresident)k
c) 23571.84user 3383.65system 31:36.83elapsed 1421%CPU (0avgtext+0avgdata 919068maxresident)k

For the overall build and sstate directories, du -s shows:
a)
3992588   build-pre-rss/sstate-cache
30804484  build-pre-rss/tmp
b)
4013272   build-with-rss/sstate-cache
36519084  build-with-rss/tmp
c)
4014744   build-with-rss2/sstate-cache
35336960  build-with-rss2/tmp

However more worryingly:

$ du -s build-pre-rss/tmp/sysroots/
2506092 build-pre-rss/tmp/sysroots/
$ du -s build-with-rss/tmp/sysroots-components/
3790712 build-with-rss/tmp/sysroots-components/
$ du -s build-with-rss2/tmp/sysroots-components/
2467544 build-with-rss2/tmp/sysroots-components/

These numbers *should* be equivalent but as you can see, b) is ~1.2GB larger. The reason turned out
to be patchelf. Taking a specific binary from a specific recipe, bc from bc-native, in a) its 82kb
(stripped) yet in b) its 2.17MB.

$ ./patchelf --set-interpreter /bin/rp bc
warning: working around a Linux kernel bug by creating a hole of 2084864 bytes in ‘bc’

https://github.com/NixOS/patchelf/blob/master/src/patchelf.cc#L710 shows that this "hole" is just
padded zeros using memset, its not a proper sparse hole.

This patch copies files with cp --sparse=always after modifying them with patchelf, then replacing
the original file. The better fix will be to fix this in patchself itself and seek() there
when writing the new file but that means new uninative tarballs and will take a bit of work
so I'm proposing this workaround in the meantime.

Also, this patch drops error handling since subprocess check_output() tracebacks will print this
information if the command fails so we can simplify the code.

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