[GlobalISel] make library an optional component

Summary:
Mark the LLVMGlobalISel library as optional in
LLVMBuild.txt, since the library is only built
if LLVM_BUILD_GLOBAL_ISEL is set. Without doing
this, llvm-config includes the library in the
list of components regardless of whether it's
built, and then will error out when asked for
the library names/paths.

Reviewers: qcolombet

Subscribers: joker.eph, llvm-commits, vkalintiris

Differential Revision: http://reviews.llvm.org/D16386

llvm-svn: 258379