build: Enable "fat" LTO for release builds

[larseggert]

Because that's what Firefox does:
https://searchfox.org/mozilla-central/source/config/makefiles/rust.mk#95

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.48%. Comparing base (83e6733) to head (b248127).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1751   +/-   ##
=======================================
  Coverage   89.48%   89.48%           
=======================================
  Files         126      126           
  Lines       38866    38866           
=======================================
  Hits        34780    34780           
  Misses       4086     4086           

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[github-actions]

Benchmark results

Performance differences relative to 2750423.

• coalesce_acked_from_zero 1+1 entries
  time: [192.15 ns 192.66 ns 193.22 ns]
  change: [-24.872% -24.546% -24.199%] (p = 0.00 < 0.05)
  💚 Performance has improved.

• coalesce_acked_from_zero 3+1 entries
  time: [233.63 ns 234.27 ns 234.92 ns]
  change: [-20.095% -19.819% -19.540%] (p = 0.00 < 0.05)
  💚 Performance has improved.

• coalesce_acked_from_zero 10+1 entries
  time: [234.17 ns 235.59 ns 237.47 ns]
  change: [-20.170% -17.242% -11.811%] (p = 0.00 < 0.05)
  💚 Performance has improved.

• coalesce_acked_from_zero 1000+1 entries
  time: [216.42 ns 216.80 ns 217.27 ns]
  change: [-19.522% -19.011% -18.469%] (p = 0.00 < 0.05)
  💚 Performance has improved.

• RxStreamOrderer::inbound_frame()
  time: [120.85 ms 121.01 ms 121.26 ms]
  change: [-5.0953% -4.8773% -4.6604%] (p = 0.00 < 0.05)
  💚 Performance has improved.

• transfer/Run multiple transfers with varying seeds
  time: [119.75 ms 120.03 ms 120.31 ms]
  thrpt: [33.248 MiB/s 33.325 MiB/s 33.403 MiB/s]

• transfer/Run multiple transfers with the same seed
  time: [120.81 ms 120.98 ms 121.18 ms]
  thrpt: [33.010 MiB/s 33.064 MiB/s 33.111 MiB/s]

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