Driver: add -m{,no-}long-calls support
This mirrors the GCC option for the ARM backend. This option enables the
backend option "-enable-arm-long-calls". The default behaviour is that this is
disabled due to the slight overhead of the generated calls.
If the target of jumps are greater than 64M range of offset-based jumps, then
the target address must be loaded into a register to make an indirect jump. The
backend support for this has been present, but was not previously controllable
by the proper flag.
llvm-svn: 210398