implement X86_64 asm for overflow safe mul
Merge pull request #448 from dawgfoto/asmOverflow

implement X86_64 asm for overflow safe mul
unittests with shared library

- add SHARED flag to posix.mak

- default to SHARED=1 on linux

- setting SHARED to a non-empty string implies PIC and
  builds a shared library for testing

- setting SHARED to an empty string builds a single
  executable for testing

- Add a moduleUnitTester that only runs a single unit test
  selected via a cmdline argument. This allows to run tests
  in parallel with a single executable/shared library.
restrict SHARED to linux_64
Merge pull request #447 from dawgfoto/SharedRuntime_7

unittests with shared library
improve callWithStackShell

- compatible with LDC's inline asm
- only push callee save registers
fix random crash with -cov
Merge pull request #426 from dawgfoto/callWithStackShell

improve callWithStackShell
Merge pull request #449 from WalterBright/fixcovercrash

fix random crash with -cov