Generate .build-id directory tree after every build.

The .build-id directory tree is as described here:
https://fedoraproject.org/wiki/RolandMcGrath/BuildID

Tools such as llvm-symbolizer understand this tree and can use it
to look up symbols by build ID. This CL adds a post-build step that
updates the .build-id directory under symbols after every build if
necessary to contain symlinks to the corresponding symbols file.

I also considered adding this as a build step after copying the symbol
file to the symbols directory. However this would be complicated
by the fact that many binaries have the same build-id such as the
various copies of libc++ for the different APEXes. As a result it
would be difficult to implement a .build-id updater that produces a
correct symlink tree while taking into account concurrent updates to
the same symlink by different build steps. This situation is resolved
in the .build-id directory generator by having the symlink point to
the lexically smallest path out of all paths with the same build-id.

I measured the performance of the UpdateBuildIdDir function by writing
a standalone program that just calls the function and running it over
a copy of the symbols directory for a specific target. On my machine
the execution time of the program when simulating various scenarios
was as follows:

Null build [1]:             36.2 ms ±   3.4 ms
Initial build [2]:         162.3 ms ±   6.6 ms
Single update [3]:         128.0 ms ±   6.1 ms
Invalid .build-id dir [4]: 222.6 ms ±   8.6 ms

This is with some improvements that have been contributed to the Go
standard library [5,6]; without those improvements a null build is
37.9 ms ± 4.4 ms and a single update is 143.9 ms ± 4.5 ms.

[1] hyperfine './update-build-id ~/2/test-symbols2/'
[2] hyperfine -p 'rm -rf ~/2/test-symbols2/.build-id' './update-build-id ~/2/test-symbols2/'
[3] hyperfine -p 'dd if=/dev/urandom of=$HOME/2/test-symbols2/system/bin/init conv=notrunc seek=808 bs=1 count=16' './update-build-id ~/2/test-symbols2/'
[4] hyperfine -p 'touch ~/2/test-symbols2/.build-id/corrupt; sleep 0.1; touch ~/2/test-symbols2/system/bin/init' './update-build-id ~/2/test-symbols2/'
[5] https://go.dev/cl/570877
[6] https://go.dev/cl/571436

Bug: 328702178
Change-Id: I8fc0ea81bd31ec80d6b912ba477e2e24b6b05f68