Wrong expansion of $PLATFORM by ld.so on 18.04

[rojer]

It seems that expansion of the $PLATFORM dynamic linker variable has changed between xenial and bionic:

rojer@xenial$ apt-cache policy libc6
libc6:
  Installed: 2.23-0ubuntu10
  Candidate: 2.23-0ubuntu10
rojer@xenial$ LD_PRELOAD='/foo/$LIB/$PLATFORM/bar.so' strace -f /bin/true 2>&1 | grep '^open.*foo'
open("/foo/lib/x86_64-linux-gnu/x86_64/bar.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)

rojer@bionic$ apt-cache policy libc6
libc6:
  Installed: 2.27-3ubuntu1
  Candidate: 2.27-3ubuntu1
rojer@bionic$ LD_PRELOAD='/foo/$LIB/$PLATFORM/bar.so' strace -f /bin/true 2>&1 | grep '^open.*foo'
openat(AT_FDCWD, "/foo/lib/x86_64-linux-gnu/haswell/bar.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)

What was `x86_64` before has changed to `haswell` which is way too specific and also doesn't match the man page for ld.so which clearly states:

       $PLATFORM (or equivalently ${PLATFORM})
              This expands to a string corresponding to the processor type of the host system (e.g., "x86_64"). On some architectures, the Linux kernel doesn't provide a platform string to the dynamic linker. The value of this string is taken
              from the AT_PLATFORM value in the auxiliary vector (see getauxval(3)).

I verified that `getauxval(AT_PLATFORM)` still returns `x86_64`.