This patch implements 64-bit shifts in assembly code. Previously, generic C
library code from libgcc would be used to perform the shifts, which was much
more costly in terms of code size.

I observed 700 PASS->FAIL regressions from the GCC testsuite alone when these
64-bit shifts were implemented incorrectly, hence I've assumed there is
already adequate test coverage that shifts operate correctly, and I have not
added new tests to verify their correct execution.

For the following program, the below code size reduction is observed:
  long long a;

  int
  main (void)
  {
    a = a >> 4;
    return 0;
  }

With shift patch 3:
   text    data     bss     dec     hex filename
    670      12      26     708     2c4 a.out
With new patch:
   text    data     bss     dec     hex filename
    512      12      26     550     226 a.out

Ok for trunk?