Fix Giant's Knife unexpected behaviours (HarbourMasters#834)

* Giant's Knife Behaviour Fixes

Fixes a case where Giant's Knife (specifically, breaking and re-buying it) can behave unexpectedly if you don't have a Kokiri Sword in your inventory.  Also fixes the broken icon not showing up in inventory after you break it.

* less whitespace

* fix 0xE check

Accidentally committed with the wrong check

* Move the grayscale code up

Moved the grayscaling portion up higher in the function and renamed "bool not_acquired" to "bool age_restricted".  Thanks to @vaguerant for the suggestion!