gendwarfksyms: Add a separate pass to resolve FQNs

Using dwarf_getscopes_die to resolve fully-qualified names turns out to
be rather slow, and also results in duplicate scopes being processed,
which doesn't help. Simply adding an extra pass to resolve names for all
DIEs before processing exports is noticeably faster.

For the object files with the most exports in a defconfig+Rust build,
the performance improvement is consistently >50%:

rust/bindings.o: 1038 exports
before: 9.5980 +- 0.0183 seconds time elapsed ( +- 0.19% )
after: 4.3116 +- 0.0287 seconds time elapsed ( +- 0.67% )

rust/core.o: 424 exports
before: 5.3584 +- 0.0204 seconds time elapsed ( +- 0.38% )
after: 0.05348 +- 0.00129 seconds time elapsed ( +- 2.42% )
^ Not a mistake.

net/core/dev.o: 190 exports
before: 9.0507 +- 0.0297 seconds time elapsed ( +- 0.33% )
after: 3.2882 +- 0.0165 seconds time elapsed ( +- 0.50% )

rust/kernel.o: 129 exports
before: 6.8571 +- 0.0317 seconds time elapsed ( +- 0.46% )
after: 2.9096 +- 0.0316 seconds time elapsed ( +- 1.09% )

net/core/skbuff.o: 120 exports
before: 5.4805 +- 0.0291 seconds time elapsed ( +- 0.53% )
after: 2.0339 +- 0.0231 seconds time elapsed ( +- 1.14% )

drivers/gpu/drm/display/drm_dp_helper.o: 101 exports
before: 1.7877 +- 0.0187 seconds time elapsed ( +- 1.05% )
after: 0.69245 +- 0.00994 seconds time elapsed ( +- 1.44% )

net/core/sock.o: 97 exports
before: 5.8327 +- 0.0653 seconds time elapsed ( +- 1.12% )
after: 2.0784 +- 0.0291 seconds time elapsed ( +- 1.40% )

drivers/net/phy/phy_device.o: 95 exports
before: 3.0671 +- 0.0371 seconds time elapsed ( +- 1.21% )
after: 1.2127 +- 0.0207 seconds time elapsed ( +- 1.70% )

drivers/pci/pci.o: 93 exports
before: 1.1130 +- 0.0113 seconds time elapsed ( +- 1.01% )
after: 0.4848 +- 0.0127 seconds time elapsed ( +- 2.63% )

kernel/sched/core.o: 83 exports
before: 3.5092 +- 0.0223 seconds time elapsed ( +- 0.64% )
after: 1.1231 +- 0.0145 seconds time elapsed ( +- 1.29% )

Overall, a defconfig+DWARF5 build with gendwarfksyms and Rust is 14.8%
faster with this patch applied on my test system. Without Rust, there's
still a 10.4% improvement in build time when gendwarfksyms is used.

Note that symbol versions are unchanged with this patch.

Suggested-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>