arm64: Use aarch64elf and aarch64elfb emulation mode variants

The aarch64linux and aarch64linuxb emulation modes are not supported by
bare-metal toolchains and Linux using them forbids building the kernel
with these toolchains.

Since there is apparently no reason to target these emulation modes, the
more generic elf modes are used instead, allowing to build on bare-metal
toolchains as well as the already-supported ones.

Fixes: 3d6a7b99

 ("arm64: ensure the kernel is compiled for LP64")

Cc: stable@vger.kernel.org
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>