Allow -target= and --target options

Using clang as a cross-compiler with the 'target' option could be confusing
for those inexperienced in the realm of cross compiling.

This patch would allow the use of all these four variants of the target option:
-target <triple>
--target <triple>
-target=<triple>
--target=<triple>

Patch by Gabor Ballabas.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@231787 91177308-0d34-0410-b5e6-96231b3b80d8