Revert "[gdb] Fix warning in foreach_arch selftests"

Revert "[gdb] Fix warning in foreach_arch selftests"

This reverts commit fc18b1c5afd ("[gdb] Fix warning in foreach_arch
selftests").

The commit introduced regressions for an --enable-targets=all build:
...
Running selftest print_one_insn::A6.^M
Self test failed: Cannot access memory at address 0x0^M
...
and while investigating those I realized that the commit fc18b1c5afd
complicates things by trying to set the current osabi.

So, revert the patch in preparation for a simpler solution.

Tested on x86_64-linux.