buildroot

kvm-unit-tests: Fix x86_64 to use host compiler

kvm-unit-tests requires a multilib compiler for x86_64 as it compiles
32bit boot code.

This patch uses the BR2_HOSTARCH_NEEDS_IA32_COMPILER option to enforce
this and avoids using TARGET_CROSS for x86_64 targets and uses the host
32bit (and 64bit) capable compiler.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
[Thomas: reword comment in .mk file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>