arm32: Fix rodata-merge-map

arm32: Fix rodata-merge-map

the test expects a second, but useless, $d mapping symbol for
the partially merged section, and specifically disallows one
for the completely merged section.  The new merging algorithm
makes it so that also the partially merged sections are conceptually
SEC_EXCLUDED, except the first merge section (e.g. as if the very
first object file already contains all strings).  So that second mapping
symbol is now missing.  It never was needed anyway.

So, adjust the test.