[fuchsia_clang] Use ACTION_NAMES struct instead of strings.

A first CL [1] replaced the use of string literals within
cc_toolchain_config_template.bzl with ACTION_NAMES struct
fields, while a second CL [2] modified the same file to
add a new feature definition (using string literals).

This CL harmonizes the definition by switching the recent
string literals from [2] into ACTION_NAMES field names
too.

[1] https://fuchsia-review.git.corp.google.com/c/sdk-integration/+/786870?usp=dashboard
[2] https://fuchsia-review.git.corp.google.com/c/sdk-integration/+/786962?usp=dashboard

Bug: None
Change-Id: I1468b6e4a4137a1932e9b4f2fd1d98e0af82ed68
Reviewed-on: https://fuchsia-review.googlesource.com/c/sdk-integration/+/786963
Reviewed-by: Renato Mangini Dias <mangini@google.com>
Commit-Queue: David Turner <digit@google.com>