elf: Limit tst-prelink-cmp target archs

elf/tst-prelink-cmp was initially added for x86 (commit fe534fe8) to validate
the fix for Bug 19178, and later applied to all architectures that use GLOB_DAT
relocations (commit 89569c8b).  However, that bug only affected targets that
handle GLOB_DAT relocations as ELF_TYPE_CLASS_EXTERN_PROTECTED_DATA, so the test
should only apply to targets defining DL_EXTERN_PROTECTED_DATA, which gates the
usage of the elf type class above.  For all other targets not meeting that
criteria, the test now returns with UNSUPPORTED status.

Fixes the test on POWER10 processors, which started using R_PPC64_GLOB_DAT.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>