patchelf.cc: handle DT_MIPS_XHASH and .MIPS.xhash

[ghost]

glibc changed their ABI in commit 23c1c256ae7b0f010d0fcaff60682b620887b164 on 2019-Aug-29, by changing the structure of the .gnu.hash data on MIPS and moving it to a different section. We need to adapt to this change by glibc.

Closes #368

[ghost]

This fixes #368 on my testing setup.

Will reverify once I confirm that it fixes the QEMU-VM test case in that PR, but I'm pretty darn sure it will work.

[ghost]

This bug has been sufficiently frustrating to track down that I want to make sure I've completely documented how to both (a) reproduce it and (b) verify that it is fixed. I'm currently running the commands below (which require quite a lot of rebuilding); as soon as they finish sometime tonight I will undraftify this PR. Thank you for your patience.

Instructions for verifying this bug, and the fact that it is indeed fixed (based on #368):

# Assumes you have set $NIX_PATH explicitly.  If not, do so.
cd $NIX_PATH/nixpkgs
git fetch https://github.com/a-m-joseph/nixpkgs patchelf-issue-368 # e642d58dd59d40d8b2062b5ac33d053aab37cfc6
git switch -c patchelf-issue-368 FETCH_HEAD

cd /tmp  # or elsewhere
git clone -b patchelf-issue https://github.com/a-m-joseph/mips64-nixpkgs-qemu # 69126a995bc8b6a2f33b80fd5cb456acdf8a6a07
cd mips64-nixpkgs-qemu
nix-shell shell.nix --argstr NIXPATH $NIX_PATH

Once the VM boots, type ./demo.sh and you should get a crash with Inconsistency detected by ld.so ... (bitmask_nwords & (bitmask_nwords - 1)) == 0 failed.

Then, shut down the VM, cherry-pick NixOS/nixpkgs@12618e7 into your $NIX_PATH/nixpkgs and run the nix-shell command above again; the bootstrap-tools should unpack correctly with no crash.

[ghost]

Yep, it works, and the steps above reproduce the problem and verify the fix.

[klemensn]

Yep, it works, and the steps above reproduce the problem and verify the fix.

I have not reproduced your issue due to limited resources but the fix makes sense.

[ghost]

Ping...