[ELF] - Add test case for checking PT_INTERP behavior.

When PT_INTERP is specified in PHDRS command, it should be created.
(if other conditions met)

We had no test for the folowing line:
https://github.com/llvm-mirror/lld/blob/master/ELF/LinkerScript.cpp#L1108
And for this header itself.

Patch fixes that.

llvm-svn: 336359