glibc: Install AArch64 loader link correctly for usrmerge+multilib

The AArch64 little-endian ABI requires that the dynamic loader is always
available at /lib/ld-linux-aarch64.so.1. Similarly, the big-endian ABI
requires that the dynamic loader is always available at
/lib/ld-linux-aarch64_be.so.1.

glibc-package.inc contains code that tries to ensure this, but
unfortunately it is defeated by the combination of multilib and usrmerge
because it does not take into account that /lib is the same as /usr/lib
with usrmerge when it adds the loader path to libc_baselibs and when it
attempts to show that /usr/lib is empty in do_poststash_install_cleanup.
This results in the symlink not being included in the package and a build
failure due to rmdir failing.

Richard Purdie also suggested[1] that ${nonarch_base_libdir} should not be
used as a synonym for /lib in this case.

This hopefully-fixed version always sets ARCH_DYNAMIC_LOADER and then uses
${root_prefix}/lib/${ARCH_DYNAMIC_LOADER} to refer to the dynamic loader
which works with both multilib and usrmerge. Since ARCH_DYNAMIC_LOADER is
only non-empty if the symlink is required, the code to create it can move
to do_install_append. Then do_poststash_install_cleanup needs to be taught
that ${exec_prefix}/lib may not be empty if the dynamic loader symlink is
there.

It appears not to be possible to specify the name of the loader via a
variable with an override, since the _aarch64 override is applied even for
_aarch64-be, so I've set the loader name using ${TARGET_ARCH} instead.

Build-tested and inspected core-image-minimal rootfs with:

* AArch64 no multilib (real loader in correct place)
  MACHINE = "qemuarm64"

* AArch64 multilib (symlink in correct place)
  MACHINE = "qemuarm64"
  MULTILIBS = "multilib:lib32"
  DEFAULTTUNE_virtclass-multilib-lib32 = "armv7at-neon"
  require conf/multilib.conf

* AArch64 usrmerge (real loader in correct place)
  DISTRO_FEATURES += "usrmerge"
  MACHINE = "qemuarm64"

* AArch64 multilib usrmerge (symlink in correct place)
  DISTRO_FEATURES += "usrmerge"
  MACHINE = "qemuarm64"
  MULTILIBS = "multilib:lib32"
  DEFAULTTUNE_virtclass-multilib-lib32 = "armv7at-neon"
  require conf/multilib.conf

* big-endian versions of all of the above by also setting
  DEFAULTTUNE = "aarch64_be".
  (building glibc only.)

* x86_64 (real loader in /lib as before)[2]
  MACHINE = "qemux86"

* x86_64 multilib (real loader in /lib64 as before)
  MACHINE="qemux86-64"
  MULTILIBS = "multilib:lib32"
  DEFAULTTUNE_virtclass-multilib-lib32 = "x86"
  require conf/multilib.conf

I also tested leaving an unwanted file in ${exec_prefix}/lib for
do_poststash_install_cleanup to detect, and I believe the detection always
worked correctly.

[1] http://lists.openembedded.org/pipermail/openembedded-core/2018-November/276120.html

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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