Fix typos in PCG modules

[schmouk]

• in pcg128_64.py, line 145, add parenthesis around last term of operation |.
• in pcg1024_32.y, lines 261 and 262, remove erroneous comments at end of lines.
• in pcg1024_32.y, rename extendedstep with externalstep.

[schmouk]

Fully completed.
TestED tested.