Fix $LIB in glibc

DT_RUNPATH, DT_RPATH, LD_PRELOAD, LD_AUDIT, etc. can use `$LIB` which
is expanded to the relative path to libdir from prefix.  However glibc
was expanding `$LIB` as e.g. `x86-64-linux-gnu` instead of
`lib/x86-64-linux-gnu`.

Fixes #874
10 jobs for valentindavid/glibc-dl-dst-lib in 229 minutes and 4 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Flatpak
passed #294561417
aarch64
app_aarch64

02:30:30

passed #294561421
armhf
app_arm

00:58:50

passed #294561416
cache_i686 x86_64
app_i686

03:46:31

passed #294561424
ppc64le allowed to fail
app_powerpc64le

01:17:20

passed #294561411
cache_x86_64 x86_64
app_x86_64

01:16:53

 
  Vm
manual #294561437
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #294561439
armhf
minimal_systemd_vm_arm

00:02:32

passed #294561433
cache_i686 x86_64
minimal_systemd_vm_i686

00:02:09

manual #294561441
ppc64le allowed to fail manual
minimal_systemd_vm_powerpc64le
passed #294561428
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:01:17