For reference I looked up the default provided machine conf files inside
poky/meta.

Following seems to build qemuarm for 32bit, i.e. both kernel and user space
32bit
*
http://git.yoctoproject.org/cgit/cgit.cgi/poky/tree/meta/conf/machine/qemuarm.conf?h=jethro

And following seems to build qemuarm64 for 64 bit, i.e. both kernel and
user space 64bit
*
http://git.yoctoproject.org/cgit/cgit.cgi/poky/tree/meta/conf/machine/qemuarm64.conf?h=jethro

Please guide me how can I define a machine where kernel is built 64bit but
the user space is 32bit.
I attempted a few things by trying to override the TUNE flags but I am not
able to get it right.

Regards,
Sourabh