fix(squash): keep ld cache under initdir

fix(squash): keep ld cache under initdir

When running kdump on PowerPC, the following bug is hit:
[    0.391629] Freeing unused kernel memory: 5568K
[    0.391634] This architecture does not have kernel memory protection.
/bin/sh: error while loading shared libraries: libc.so.6: cannot open shared object file: No such file or directory
[    0.392214] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00007f00
[    0.392214]
[    0.392223] CPU: 6 PID: 1 Comm: init Not tainted 4.18.0-319.el8.ppc64le #1
[    0.392228] Call Trace:
[    0.392234] [c00000000c703c10] [c000000008ecb94c] dump_stack+0xb0/0xf4 (unreliable)
[    0.392243] [c00000000c703c50] [c000000008167324] panic+0x148/0x3c4
[    0.392249] [c00000000c703cf0] [c000000008170474] do_exit+0xcd4/0xd40
[    0.392255] [c00000000c703dc0] [c0000000081705b0] do_group_exit+0x60/0x110
[    0.392261] [c00000000c703e00] [c000000008170684] sys_exit_group+0x24/0x30
[    0.392268] [c00000000c703e20] [c00000000800b408] system_call+0x5c/0x70

This is due to the non-conventional library path:
ldd /bin/bash
        linux-vdso64.so.1 (0x00007fffbdc90000)
        libtinfo.so.6 => /lib64/libtinfo.so.6 (0x00007fffbda80000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007fffbda50000)
        libc.so.6 => /lib64/glibc-hwcaps/power9/libc-2.28.so (0x00007fffbd830000)
                            ^^^
        /lib64/ld64.so.2 (0x00007fffbdcb0000)

ldd finds the path by libc.so.6 -> /usr/lib64/libc.so.6 ->
/usr/lib64/glibc-hwcaps/power9/libc-2.28.so and cache the result. So
when dracut_install, it only saw
'/usr/lib64/glibc-hwcaps/power9/libc-2.28.so' and blind to
'/usr/lib64/libc.so.6'

In the final kdumpimg, the symlink /usr/lib64/libc.so.6 is not created,
hence ld.so can not find the /usr/lib64/glibc-hwcaps/power9/libc-2.28.so

On the other hand, during the process of building kdumpimg, all of dynamic library info
have been cached in ld.so.cache. Hence this bug can be simplely resolved
by keeping ld cache under $initdir/etc.

Signed-off-by: Pingfan Liu <piliu@redhat.com>
---
v3 -> v4:
  use inst() instead of copy
v2 -> v3:
  fix format by shfmt -s -w modules.d/99squash/module-setup.sh
v1 -> v2:
  cp -r /etc/ld.so* instead of move, since after switch-root, initdir
can not be seen any longer