TestMultithreaded improvements

These tests link against host lldb API. Compiler's target triple
must match liblldb triple. Instead of naively skipping i386, I added
a check of the liblldb arch against the compiler target arch.

This is useful for 32 bit API builds (planned for Windows)

Since remote is disabled, we can assume the os is the same.

Also, removed skipIfLinuxClang because it's passing

llvm-svn: 236396