Delay base computation for ELF files until we have a sample address.

[gmarin13]

We can use the sample's offset into the mapping to differentiate between small
segments that are in the same file page when associating a segment with the
mapping.

[codecov-io]

Codecov Report

Merging #623 (4b38d16) into master (7c2eacd) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #623      +/-   ##
==========================================
- Coverage   66.94%   66.85%   -0.10%     
==========================================
  Files          39       39              
  Lines        7195     7226      +31     
==========================================
+ Hits         4817     4831      +14     
- Misses       1942     1954      +12     
- Partials      436      441       +5     

Impacted Files	Coverage Δ
...ithub.com/google/pprof/internal/elfexec/elfexec.go	40.62% <0.00%> (-2.86%)	⬇️
...hub.com/google/pprof/internal/binutils/binutils.go	46.23% <0.00%> (+1.21%)	⬆️

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[gmarin13]

Updated the code. PTAL.

[gmarin13]

All the checks pass after the Golang runtime fix.