Fix some tests for PPC64le architecture

Summary:
- Fix test jump for powerpc64le
  Jumping directly to the return line on power architecture dos not means
  returning the value that is seen on the code. The last test fails, because
  it needs the execution of some assembly in the beginning of the function.
  Avoiding this test for this architecture.

- Avoid evaluate environ variable name on Linux
  On Linux the Symbol environ conflicts with another variable, then in
  order to avoid it, this test was moved into a specific test, which is not
  supported if the OS is Linux.

- Added PPC64le as MIPS behavior
  Checking the disassembler output, on PPC64le machines behaves as MPIS.
  Added method to identify PPC64le architecture and checking it when
  disassembling instructions in the test case.

Reviewers: labath

Reviewed By: labath

Subscribers: clayborg, labath, luporl, alexandreyy, sdardis, ki.stfu, arichardson

Differential Revision: https://reviews.llvm.org/D44101
Patch by Leonardo Bianconi <leonardo.bianconi@eldorado.org.br>.

llvm-svn: 327977