arm64: setup: report ELF_PLATFORM as the machine for utsname

arm64: setup: report ELF_PLATFORM as the machine for utsname

uname -m reports the machine field from the current utsname, which should
reflect the endianness of the system.

This patch reports ELF_PLATFORM for the field, so that everything appears
consistent from userspace.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 94ed1f2cb5d46533f10262b1b760db7dbec9cf10)

Signed-off-by: Victor Kamensky <victor.kamensky@linaro.org>