Make .s files generate cflags for ninja

Make .s files generate cflags for ninja

With toolchains that use cc as an assembler cflags are expected to get
passed along to the assembler as well.  In the ninja build writer this
only happens if the source set contains .asm, .c, .cpp, ... but not .s
files.

While we could set this up to map through the tool type map, that limits
future flexibility too.

This doesn't seem to affect chromium until arm platforms are compiled.

BUG=507943
TEST=https://code.google.com/p/chromium/issues/detail?id=507943#c1
R=brettw@chromium.org

Committed: https://crrev.com/3f87569bb10ffc8a925ed75147b783d089a2bc4f
Cr-Commit-Position: refs/heads/master@{#337891}

[Peter Mayo, 2015-07-08 02:56:22]

PTAL

[Dirk Pranke, 2015-07-08 19:37:39]

dpranke@chromium.org changed reviewers:
+ dpranke@chromium.org

[Dirk Pranke, 2015-07-08 19:37:39]

lgtm

[Peter Mayo, 2015-07-08 19:43:31]

The CQ bit was checked by petermayo@chromium.org

[commit-bot: I haz the power, 2015-07-08 19:44:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1222713012/1

[commit-bot: I haz the power, 2015-07-08 20:00:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-08 20:01:00]

Patchset 1 (id:??) landed as
https://crrev.com/3f87569bb10ffc8a925ed75147b783d089a2bc4f
Cr-Commit-Position: refs/heads/master@{#337891}