csprng: fix undefined behavior in entropy pool initialization

csprng: fix undefined behavior in entropy pool initialization

Fixed cppcheck error: shifting 64-bit value by 504 bits is undefined.
The TSC has only 64 bits (8 bytes), but the loop was iterating 64 times.
Changed to extract only 8 bytes from TSC and fill remaining pool with
tick count variations.

Test: make test-all PASS (131/131 smoke, 38/38 analyzer, 111/111 host)