Allow the user to set the LLVMINTERP environment variable as a workaround, for

when they have to run a gccld shell script without having lli in their path.
This is intended to address Bug 289.

Also, emit the traditional syntax ${1+"$@"} for passing all of a shell script's
args to a subprocess. If you have arguments that have spaces in them, $* will
not preserve the quoting (i.e., the quoted string "foo bar" as an argument will
end up as two arguments "foo" "bar" to lli.)

llvm-svn: 13414