Add a --show-percs option to cg_annotate and callgrind_annotate.

Add a --show-percs option to cg_annotate and callgrind_annotate.

Because it's very useful. As part of this, the "percentage of events
annotated" numbers at the bottom of the output is changed to "events
annotated" so that --show-percs doesn't compute a percentage of a
percentage.

Example output lines:
```
4,967,137,442 (100.0%)  PROGRAM TOTALS

4,543 (25.23%)    17,566 ( 0.43%)    47,993 ( 0.92%) /build/glibc-OTsEL5/glibc-2.27/elf/dl-lookup.c

1 ( 0.01%) 2,000,001 (49.29%) 3,000,004 (57.36%)     for (int i = 0; i < 1000000; i++) {
```

The commit also adds some much-needed tests for cg_annotate and
callgrind_annotate.