gdb.dwarf2: Fix fission-dw-form-strx.exp workflow

The test was using gdb_compile_shlib to create the .dwo file, which
runs the linker. This is incorrect for DWARF fission - .dwo files
should be created by objcopy extraction, not linking.

Per commit 6a29913eeb9, "it was a bad idea to generate a .dwo file
using the linker, since the idea behind .dwo files is that they do
not need to be linked."

GCC's assembler doesn't set the SHF_EXCLUDE flag on .debug_*.dwo
sections, so the linker preserves them in the output. Clang's
assembler sets SHF_EXCLUDE, causing the linker to exclude these
sections. Both approaches are valid, but only the GCC approach
happens to work with this test's incorrect use of the linker.

Rewrite the test to use build_executable_and_dwo_files with the
split-dwo option, matching all other fission-*.exp tests. This uses
objcopy to properly extract .dwo sections without involving the
linker.

Changes:
- Use build_executable_and_dwo_files instead of gdb_compile_shlib
- Generate both skeleton and DWO CUs in single Dwarf::assemble block
- Add debug_str_offsets section for DW_FORM_strx support
- Add is_remote host check (required for objcopy-based workflow)
- Fix DW_AT_dwo_name to match generated filename

Tested: GCC 13.3.0, Clang-23 (passes)