tilegx: Set this_hdr.sh_entsize only if section size > 0

tilegx: Set this_hdr.sh_entsize only if section size > 0

This fixes:

/export/build/gnu/binutils-cross/build-tilegx-linux/ld/ld-new   -o tmpdir/comm-data -z norelro -L/export/gnu/import/git/sources/binutils-gdb/ld/testsuite/ld-elf  -T comm-data2.ld -Ltmpdir -lcomm-data tmpdir/comm-data2.o
sh: line 1:  9208 Segmentation fault      (core dumped) /export/build/gnu/binutils-cross/build-tilegx-linux/ld/ld-new -o tmpdir/comm-data -z norelro -L/export/gnu/import/git/sources/binutils-gdb/ld/testsuite/ld-elf -T comm-data2.ld -Ltmpdir -lcomm-data tmpdir/comm-data2.o 2>&1
FAIL: Common symbol override test

	* elfxx-tilegx.c (tilegx_elf_finish_dynamic_sections): Set
	this_hdr.sh_entsize only if section size > 0.