Fix regression from gdbarch registry change

Fix regression from gdbarch registry change

The gdbarch registry patch introduced a regression that could cause a
crash when opening files in gdb.  The bug is that, previously, the
solib ops would default to current_target_so_ops; but the patch
changed this code to default to nullptr.  This patch fixes the bug by
reintroducing the earlier behavior.  This is PR gdb/29449.

I managed to reproduce the bug with a riscv-elf build and then
verified that this fixes the problem.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29449