Use a GN arg for controlling linker map file generation.

Map files aren't generated on Linux by default now that linker map file
generation is hooked into !use_lld (https://crbug.com/716209).

This CL adds a GN arg, enable_linker_map, that defaults to true for
Android and makes it easy to generate map files on Linux.

BUG=717550,716209

Review-Url: https://codereview.chromium.org/2888623003
Cr-Commit-Position: refs/heads/master@{#472834}
3 files changed