chromium68: custom toolchain, target v8 snapshot

chromium68: custom toolchain, target v8 snapshot

Modify the chromium68 recipe to make it more like the one available in
meta-browser: https://github.com/OSSystems/meta-browser

Instead of passing Yocto's build flags to the cros (=ChromeOS)
toolchain, generate a toolchain definition file specifically for this
build.

For the generation of the V8 memory snapshot, build the required files
for the target architecture, then use qemu-native to run them.
Upstream chromium addresses this problem by downloading a specific
native toolchain for every target platform, which we cannot do in this
context.

The toolchain changes trigger an issue with ARMv7 builds; backported
one patch to address that specific issue.

This changes also triggered a problem with 32 bit targets, the
generated binaries were too big. Forced the DEBUG_ARGS to use -g1 (or
-g0 otherwise).

Bug-AGL: SPEC-2514

Change-Id: Ib18431b628415c58a3c29595bfff10057e355a4b
Signed-off-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>