arm64: Move ELF_ET_DYN_BASE to 4GB / 4MB

Now that explicitly executed loaders are loaded in the mmap region, we
have more freedom to decide where we position PIE binaries in the address
space to avoid possible collisions with mmap or stack regions.

For 64-bit, align to 4GB to allow runtimes to use the entire 32-bit
address space for 32-bit pointers. On 32-bit use 4MB, to match ARM. This
could be 0x8000, the standard ET_EXEC load address, but that is needlessly
close to the NULL address, and anyone running arm compat PIE will have an
MMU, so the tight mapping is not needed.

Cc: stable@vger.kernel.org
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>