[DWARFLinkerParallel][PowerPC] Cure tests failing on powerpc machine.

location-expression.test and tls-variable.test failed because wrong
endianness was used on powerpc machine. To fix the issue this patch
uses endianness from debug map as endianness for the whole target file.
Previosly, architecture endianness won over what is specified
in debug map.

Differential Revision: https://reviews.llvm.org/D159349