mm: Revert x86_64 and arm64 ELF_ET_DYN_BASE base

Moving the x86_64 and arm64 PIE base from 0x555555554000 to 0x000100000000
broke AddressSanitizer. This is a partial revert of:

  commit eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
  commit 02445990a96e ("arm64: move ELF_ET_DYN_BASE to 4GB / 4MB")

The AddressSanitizer tool has hard-coded expectations about where
executable mappings are loaded. The motivation for changing the PIE
base in the above commits was to avoid the Stack-Clash CVEs that
allowed executable mappings to get too close to heap and stack. This
was mainly a problem on 32-bit, but the 64-bit bases were moved too,
in an effort to proactively protect those systems (proofs of concept
do exist that show 64-bit collisions, but other recent changes to fix
stack accounting and setuid behaviors will minimize the impact).

The new 32-bit PIE base is fine for ASan (since it matches the ET_EXEC
base), so only the 64-bit PIE base needs to be reverted to let x86 and
arm64 ASan binaries run again. Future changes to the 64-bit PIE base on
these architectures can be made optional once a more dynamic method for
dealing with AddressSanitizer is found. (e.g. always loading PIE into
the mmap region for marked binaries.)

Reported-by: Kostya Serebryany <kcc@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>