[gdb/testsuite] Fix options arg handling in compile_jit_elf_main_as_so

[gdb/testsuite] Fix options arg handling in compile_jit_elf_main_as_so

In commit 80ad340c902 ("[gdb/testsuite] use -Ttext-segment for jit-elf tests")
the following change was made:
...
 proc compile_jit_elf_main_as_so {main_solib_srcfile main_solib_binfile options} {
-    set options [concat $options debug]
+    global jit_load_address jit_load_increment
+
+    set options [list \
+       additional_flags="-DMAIN=jit_dl_main" \
+       additional_flags=-DLOAD_ADDRESS=$jit_load_address \
+       additional_flags=-DLOAD_INCREMENT=$jit_load_increment \
+       debug]
...

Before the change, the options argument was used, but after the change not
anymore.

Fix this by reverting back to using "set options [concat $options ...]".

Fixing this gets us twice the -DMAIN=jit_dl_main bit, once from a caller, and
once from compile_jit_elf_main_as_so.  Fix this by removing the bit from
compile_jit_elf_main_as_so, which makes the code similar to compile_jit_main.

Tested on x86_64-linux.