Use getrandom() syscall on Linux 3.17+.

glibc is lagging the kernel and hasn't yet added a syscall wrapper,
so call via the syscall number for now.
random: Use PCG instead of Tausworthe or xorshift.

xorshift is not much worse but PCG is somewhat superior.  Tausworthe is
a lot worse than PCG.

x64 or x86 use hand-written asm to handle 128-bit operations.
Pregenerated object files are provided for these platforms, built from
the fasm source.  These files are trivial to build with "fasm
source.asm".

Platforms without asm are supported with two 64-bit PCG generators.