adds gdb-index option for linker
adds gdb-index option for linker

This cl adds a new configuration flag to gold linker: "gdb_index"

The flag is on by default if is_desktop_linux && is_debug && is_component_build

The previous attempt to add this flag to all the builds was reverted
because of disk-space requirements. This checkin makes it a default
only for linux debug builds.

Why do this?

gold linker has a flag to automatically generate gdb-index section
inside shared libraries when linking.

Webkit single-file compile/linking time:

16.31s without gdb-index
16.11s with gdb-index

gdb startup time

~51s without gdb-index
~19s with gdb-index

BUG=

Committed: https://crrev.com/f68b7c07a30baac088b8eda055ba52835c04692c
Cr-Commit-Position: refs/heads/master@{#378600}

Review URL: https://codereview.chromium.org/1680943002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#383324}

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