Enable top and make other tweaks in plat/osxppc/descr

Enable top and make other tweaks in plat/osxppc/descr

David Given made top for PowerPC.  Copy the asopt phase (running top)
from linuxppc to osxppc.

Remove CC_ALIGN=-Vr to become compatible with Apple's gcc.  Apple uses
left adjustment for bitfields; the first bitfield is on the left side
(the big end), not the right side.

Remove unused variables C_LIB and OLD_C_LIB; the file libc-ansi.a
doesn't exist.

Change MACHOPT_F from -m10 to -m3.  This means to use no more than 3
adds and shifts to optimize a multiply by a constant.  I pick -m3
because -m4 can use too many instructions.  At -m4, the compiler
rewrites

    n * 14

as

    s = n << 1
    (s << 3) + (0 - s)

This means (n * 16 - n * 2), but even at ack -O6, the compiler doesn't
rewrite (a + (0 - b)) as (a - b).  The compiler emits 5 instructions:
2 of rlinmw for 2 left shifts, then addi to load 0 in a register, subf
to subtract from that 0, then add.  These 5 instructions cost 5 cycles
on the MPC7450, using the cycle counts from mach/powerpc/ncg/table.

At -m3, (n * 14) becomes 2 instructions: addi to load 14 in a register
and mullw to multiply.  This also costs 5 cycles (because mullw costs
4 cycles), but uses less space.