ICU-20526 fix pkgdata where LD_SONAME has a trailing space

ICU-20526 fix pkgdata where LD_SONAME has a trailing space

- added PKGDATA_TRAILING_SPACE to all of the pkgdataMakefile.in file.
- NOTE: Users who create their own pkgdata.inc / icupkg.inc files may need
   to recreate this PKGDATA_TRAILING_SPACE behavior.

- used the above variable, normally undefined, in mh-* files that need a trailing space

- Also, fixed use of system() in pkgdata.cpp per ICU-20538
This was causing pkgdata to return a zero status even on clang
failure, masking this issue.