Handle all combinations of GFXLIST and ROCMLIBS_GFXLIST

[saiislam]

Couple of boundary conditions were missing where only setting GFXLIST was causing an issue with the sanity checking of architecture list.

ROCMLIBS_GFXLIST defined	GFXLIST defined	Resulting GFXLIST and ROCMLIBS_GFXLIST
Yes	Yes	GFXLIST and ROCMLIBS_GFXLIST are both set to their defined values
Yes	No	GFXLIST is set to ROCMLIBS_GFXLIST, ROCMLIBS_GFXLIST remains unchanged
No	Yes	ROCMLIBS_GFXLIST is set to GFXLIST, GFXLIST remains unchanged
No	No	GFXLIST is set to a default list, ROCMLIBS_GFXLIST is set to a smaller subset of GFXLIST