Assert that page_info is 8 byte aligned

page_state assumes it can steal the bottom 3 bits.
Generate correct stubs for 0-argument syscalls

Previously, the stub generator unintentionally generated a declaration
with *no* argument types, rather than "void", but in C this means "any
arguments of any types".  Fix this to generate correct declarations
with void arguments.
Stubs for sync and fsync

This makes testgen with sync and fsync tests compile.