Speed-up absl::Symbolize by ~6x via faster file reads.

absl::Symbolize does tons of tiny reads. Speed this up by switching
from lseek+read to a pread, and by reading more data than requested
into a buffer.

A faster absl::Symbolize will be helpful in tests and when printing
stack traces on /threadz etc.

Results for absl::Symbolize benchmark that exercises uncached behavior
of absl::Symbolize:

```
name          old time/op  new time/op  delta
BM_Symbolize  16.4ms ±12%   2.6ms ± 0%  -84.06%  (p=0.001 n=5+9)
```

PiperOrigin-RevId: 582687566
Change-Id: I44caf189d81867f3fd8c050a3100a4b9a8e744d7