Rust: Use nehalem cpu-target when building with sandybridge arch variant

On Android, sandybridge doesn't include AVX support, so it's more like
nehalem target on Rust toolchain or GCC.

[cafebabe: This fixes booting on CPUs that does not support AVX, like LGA1156 ones, and LGA1150/LGA1155 Intel Celeron/Pentium]

Change-Id: I281e8f5a91271241c48336021582a42acc6d0120
Signed-off-by: hmtheboy154 <buingoc67@gmail.com>