[testsuite] Need to compile testglue with target-specific flags

[Christophe Lyon]

Hi,

When Jason recently committed his fix for PR58678, I noticed that the
newly introduced test was failing on arm-linux target.

This is because testglue.o contains relocations incompatible with
-shared which is used in the testcase.

I am preparing a testsuite patch to fix that, but I am wondering how
to add target-specific flags when compiling testglue? It does not seem
to be supported yet.

Is there already an example of a similar use case I should use?

Thanks

Christophe.