build: userspace: No merge globals

Add a compiler option to not merge globals. gen_kobject_list.py
is not capable of distinguish addresses of merged objects. The script
itself does not look wrong. The dward specification says that the
attribute DW_AT_location with opcode DW_OP_addr encodes a machine
address and whose size is the size of an address on the target machine
but for merged objects the address is longer, not clear why.

Disable global merge when userspace is enabled to avoid this problem.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>