Eliminate SetDirective, and replace it with HasSetDirective.

Default HasSetDirective to true, since most targets have it.

The targets that claim to not have it probably do, or it is
spelled differently. These include Blackfin, Mips, Alpha, and
PIC16.  All of these except pic16 are normal ELF targets, so
they almost certainly have it.

llvm-svn: 94585