Your archive command looks fine, can you try the following.
1) Get the object files in the archive/static library
ar -t libtestlib.a
2) For each object file (say foo.o) from step 1
file foo.o
This will tell you the format of the object file. If the object file was compiled for a different platform, this would cause a failure to build the index for the archive.
To correct this you would need to recompile these files.
3) For each object file from step 1, do
nm foo.o
This will list the symbols exported from the file.