Rewrite getStackTop().

The use of LDC specific intrinsics leads to better code for inline
assembler. If no inline assembler is available then an intrinsic is
used as fallback.
Add code for Linux/PPC64.
Merge branch 'ldc' into ldc-merge-2.062
Fix x86 getStackTop().
Merge branch 'ldc' into ldc-merge-2.062.