debug/ltrace: Fix HOST_OS and ar

HOST_OS really is the target OS. Allow setting it for configure
via an environment variable.
libltrace.a should have an index:
Allow ar to be set as an environment variable, and generate
an index in this lib.

Reported-by: "Guylhem Aznar" <crossgcc@guylhem.net>
Signed-off-by: "Titus von Boxberg" <titus@v9g.de>
(transplanted from f86af11138ea2b2ecd4876d9c6c24f58ba4d68ac)

 reporter_name="Alexander BIGGA"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-06/msg00031.html"

 reporter_comment=""