Make is_starboard a global variable

[niranjanyardi]

b/393160172

Set is_starboard = false globally and true for linux based cobalt and starboard toolchains.
Remove current toolchain info from is_cobalt_hermetic_build which makes it easier to be replaced by is_starboard

[niranjanyardi]

@andrewsavage1

1. i tried out putting in common.gn - didnt work.

2. I tried is_starboard false globally and is_starboard = true in the linux cobalt and starboard toolchain args - but that doesn't work

3)making is_starboard = true in the linux args.gn and then override it in the linux host toolchain works . there's a risk of random other toolchains having is_starboard = true (like roboelectric) but maybe im being too paranoid at this point.

See b/393160172 for details

[andrewsavage1]

there's a risk of random other toolchains having is_starboard = true (like roboelectric) but maybe im being too paranoid at this point.

We can just look at the value of is_starboard for all the toolchains on linux-x64x11 and linux-evergreen platform (by printing it out), that'll be enough to verify