elf: Clear entsize when clearing SEC_MERGE|SEC_STRINGS
When generating an output from input SEC_MERGE|SEC_STRINGS sections with
different entsizes, we clear the SEC_MERGE|SEC_STRINGS bits. We also need
to clear entsize.
PR ld/33291
* ldlang.c (lang_add_section): Clearing entsize when clearing
SEC_MERGE|SEC_STRINGS.
* testsuite/ld-elf/pr33291.d: New file.
* testsuite/ld-elf/pr33291a.s: Likewise.
* testsuite/ld-elf/pr33291b.s: Likewise.