Fix DWARF DebugInfo unit test errors when cross-compiling

Summary:
When building with a Default Target set we can experience issues
in the DWARF DebugInfo unit tests because:

They assume we can generate object files for the host platform.
Some tests assume the endianess of the target we are generating
DWARF for and the host match.

This patch correct these issues by ensuring the tests which
generate objects in memory are run with respect to
LVM_DEFAULT_TARGET_TRIPLE and it's endianess.

We also make sure we don't use the hosts address size for line test
and split the triple util function in DwarfUtils into a version
that takes an address size and one that doesn't.

See also for discussion:
http://lists.llvm.org/pipermail/llvm-dev/2019-March/131212.html

Patch by: daltenty

Differential Revision: https://reviews.llvm.org/D62084

llvm-svn: 362454