Revert "Change of UserLabelPrefix default value from "_" to """

This reverts commit r258504.

This commit breaks (at least) sparc-rtems -- the OS (RTEMS) used to
override UserLabelPrefix to "", despite the arch (SPARC) having set it
to "_". Now, the OS doesn't override anymore, but the arch sets it to
"_", resulting in the wrong value. I expect this probably breaks other
OSes that overrode to "" before, as well. (Clearly we have some missing
test cases, here...)

llvm-svn: 258894