Allow for specification of which JIT to run on the commandline.

`lli -march=x86' or `lli -march=sparc' will forcefully select the JIT even on a
different platform. Running lli without the -march option will select the JIT
for the platform that it's currently running on.

Pro: can test Sparc JIT (debug printing mode) on X86 -- faster to compile/link
LLVM source base to test changes.
Con: Linking lli on x86 now pulls in all the Sparc libs -> longer link time
(but X86 can bear it, right?)

In the future, perhaps this should be a ./configure option to enable/disable
target JITting...

llvm-svn: 6360