[PECOFF] Simplify /machine option handling

/machine:ebc was previously recognized but rejected. Unknown architecture
names were handled differently but eventually rejected too. We don't need
to distinguish them.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218344 91177308-0d34-0410-b5e6-96231b3b80d8