[Driver] Pass +reserve-x25 to backend if -ffixed-x25 is specified

This was accidentally omitted in r353957 breaking the Clang test.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@353960 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed