[llvm-config][GlobalISel] Canonicalize LLVM_HAS_GLOBAL_ISEL on ON/OFF.

Previously LLVM_HAS_GLOBAL_ISEL would directly get the value of
LLVM_BUILD_GLOBAL_ISEL. This could be any integer value and not just ON
and OFF. The problem is that lit.cfg was checking for ON to define that
global-isel was supported, thus if we were setting
LLVM_BUILD_GLOBAL_ISEL with an integer value, say 1, this test would
fail whereas we do build global-isel and want to test it.

llvm-svn: 276307