Do not set default value for architeture in Makefile

Fixes a regression that ARCH parameter was not properly validated.
Invalid value would default to generic 32-bit build.

No functional change.
Variable null-move value based reduction

Instead of a fixed reduction of ONE_PLY, now
Null move dynamic reduction based on value can
grow larger in case we are above beta of a value
much higher then PawnValueMg.

Note that now an eval returning VALUE_KNOWN_WIN
makes null search to drop in qsearch.

Passed both short TC:
LLR: 2.95 (-2.94,2.94) [-1.50,4.50]
Total: 26141 W: 4871 L: 4699 D: 16571

And long TC:
LLR: 2.97 (-2.94,2.94) [0.00,6.00]
Total: 33695 W: 5309 L: 5056 D: 23330

bench: 7356053
Grammar fix in MovePicker::next_move

Thanks to Lyudmil Antonov and Michel Van den Bergh
for spotting this and suggesting the fix.

No functional change.
Fix null reduction formula

Depth is already dependent on the actual value
of ONE_PLY, in particular can be expressed like:

Depth = n * ONE_PLY

And because formula is used to calculate R that is
also dependent on the value of ONE_PLY and can be
expressed like:

R = x * ONE_PLY

We don't want to divide depth by a 'ply' value but
directly by an integer number.

Spotted by sf-x

No functional change.
Reduce VALUE_KNOWN_WIN to 10000

With some positions like

8/8/8/2p2K2/1pp5/br1p1b2/2p2r2/qqkqq3 w - -

The eval score is higher than VALUE_INFINITE because
is the sum of VALUE_KNOWN_WIN plus a big material
advantage. This leads to an assert. Here are the
steps to reproduce:

Compile SF with debug=yes then do

./stockfish
position fen 8/8/8/2p2K2/1pp5/br1p1b2/2p2r2/qqkqq3 w - -
go depth 1

This patch fixes the issue in this case, but do exsist
other positions for which the patch is not enough and
we will need to limit the eval score to be sure not
overflow the limit.

Note that is not possible to increase the value of
VALUE_INFINITE because should remain within int16_t
type to be stored in a TT entry.

bench: 7356053
Merge pull request #8 from mcostalba/master

update master