Tests fail on CentOS 6.9 (binutils is 2.20 where addr2line does not support -a flag)

xref: https://github.com/golang/go/issues/23888

### What version of pprof are you using?

a74ae6fb3cd7047c79272e3ea0814b08154a2d3c

### What operating system and processor architecture are you using?

CentOS 6.9, x86_64

### What did you do?

Compiled go from source, tests failed, came to this repo, tried running the tests, was able to reproduce the exact same failure.

### What did you expect to see?

It'd be great if the tests had passed!

### What did you see instead?

The following tests failed:
```
--- FAIL: TestObjFile (0.02s)
--- FAIL: TestObjFile/fake_mapping (0.01s)
binutils_test.go:252: SourceLine: unexpected error write |1: broken pipe
--- FAIL: TestObjFile/fixed_load_address (0.01s)
binutils_test.go:252: SourceLine: unexpected error write |1: broken pipe
--- FAIL: TestObjFile/simulated_ASLR_address (0.01s)
binutils_test.go:252: SourceLine: unexpected error write |1: broken pipe
```

### Why do these tests fail?

The code at https://github.com/google/pprof/blob/a74ae6fb3cd7047c79272e3ea0814b08154a2d3c/internal/binutils/addr2liner.go#L92 assumes that the `-a` flag is available to the binary `addr2line`, which is available if the binutils version is >=2.21 ; but the latest version available on CentOS 6.9 is 2.20