debug/elf: don't use reflection in File.Symbols

debug/elf: don't use reflection in File.Symbols

cpu: AMD Ryzen 9 5950X 16-Core Processor
             |   old.txt    |               new.txt                |
             |    sec/op    |    sec/op     vs base                |
Symbols64-32   18.486µ ± 2%   4.509µ ± 24%  -75.61% (p=0.000 n=10)
Symbols32-32   17.823µ ± 3%   4.798µ ±  2%  -73.08% (p=0.000 n=10)
geomean         18.15µ        4.651µ        -74.38%

             |    old.txt    |               new.txt                |
             |     B/op      |     B/op      vs base                |
Symbols64-32   10.680Ki ± 0%   8.922Ki ± 0%  -16.46% (p=0.000 n=10)
Symbols32-32    9.773Ki ± 0%   8.570Ki ± 0%  -12.31% (p=0.000 n=10)
geomean         10.22Ki        8.744Ki       -14.41%

             |   old.txt   |              new.txt               |
             |  allocs/op  | allocs/op   vs base                |
Symbols64-32   119.00 ± 0%   45.00 ± 0%  -62.18% (p=0.000 n=10)
Symbols32-32   125.00 ± 0%   50.00 ± 0%  -60.00% (p=0.000 n=10)
geomean         122.0        47.43       -61.11%

Fixes #61534

Change-Id: I13a96467108771df31f018853df754e42100e773
Reviewed-on: https://go-review.googlesource.com/c/go/+/512395
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>