Teach clang about the armv4/armv4t distinction

When choosing a default CPU, clang used to pick ARM7TDMI (which has Thumb) even
when the more restrictive armv4 triple was specified. This should fix that.

Patch by Jeroen Hofstee.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@183905 91177308-0d34-0410-b5e6-96231b3b80d8