Replace Base() with ObjAddr() in the plugin.ObjFile interface.

[gmarin13]

Modify the plugin.ObjFile interface by replacing the "Base() uint64" method
with an "ObjAddr(addr uint64) (uint64, error)" method. The latter computes
the objdump address corresponding to the given runtime address.

The change removes direct access to the base value, but enables us to delay
computing the base until it is actually needed.

[aalexand]

@gmarin13 Looks like some tests failed on Windows.

FAIL
FAIL	github.com/google/pprof/internal/binutils	3.685s

[codecov-io]

Codecov Report

Merging #622 (0673275) into master (94a9f03) will decrease coverage by 0.04%.
The diff coverage is n/a.

❗ Current head 0673275 differs from pull request most recent head f828ff1. Consider uploading reports for the commit f828ff1 to get more accurate results

@@            Coverage Diff             @@
##           master     #622      +/-   ##
==========================================
- Coverage   66.99%   66.94%   -0.05%     
==========================================
  Files          39       39              
  Lines        7189     7195       +6     
==========================================
+ Hits         4816     4817       +1     
- Misses       1939     1942       +3     
- Partials      434      436       +2     

Impacted Files	Coverage Δ
.../github.com/google/pprof/internal/report/source.go	77.26% <0.00%> (-0.95%)	⬇️
.../github.com/google/pprof/internal/report/report.go	21.97% <0.00%> (+0.03%)	⬆️

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[gmarin13]

@gmarin13 Looks like some tests failed on Windows.

FAIL
FAIL	github.com/google/pprof/internal/binutils	3.685s

I had one typo in the test file.