Fix chpl_regexp to use the correct CHPL_TARGET_ARCH value

chpl_regexp was using the displayed target architecture, rather than the
value used in Makefiles. This would cause it to look in the wrong
directory for a re2 install on some platforms.
Merge pull request #431 from Kyle-B/kb-chpl-target-arch-re2

Fix chpl_regexp to use the correct CHPL_TARGET_ARCH value

chpl_regexp was using the displayed target architecture, rather than the value used in Makefiles. This would cause it to look in the wrong directory for a re2 install on some platforms.