GRUB warning level increase

[Vladimir 'φ-coder/phcoder' Serbinenko]

Hello, all. I have increased the warning level in GRUB. This is done for 
quicker discovery and correction of the bugs which would be otherwise 
hard to track. Mostly it enabled warnings regarding guidelines which are 
already followed by GRUB.
Most important one is -Wcast-align. x86 doesn't enforce any alignment 
but the other platforms do. And since platform-independent code is 
usually written and tested on x86, it's important to ensure that it's 
properly aligned. The invariants checked by -Wcast-align give the 
necessary guarantee as long as void * isn't involved.
Some code  is difficult to write in a way to satisfy the warning and/or 
is platform-specific, in this case #pragma exceptions can be done.
For imported code we can add -Wno-* to module cflags for less important 
(mostly stylistic) warnings.
Also if some warnings are too annoying we can drop them.

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Regards
Vladimir 'φ-coder/phcoder' Serbinenko