[yocto] Set recipe architecture different than the machine architecture

[MOHAMMAD RASIM]

Hi,

My machine.conf is including the 
`conf/machine/include/arm/arch-armv7ve.inc` so it builds everything for 
arm (32bit)  but my kernel recipe doesn't compile for 32bit arm and can 
only be compiled with aarch64 cross compiler.

So I want to set my kernel recipe to be built for aarch64 and all other 
packages for armv7.

I tried to set

PACKAGE_ARCH = "aarch64"

but the kernel still uses the arm gcc and not aarch64 gcc

Thanks