fix dyncall build on Strawberry Perl

Tested with Strawberry Perl 5.10.1.2-32bit, 5.16.0.1-32bit, 5.14.2.1-64bit as well as Cygwin.

Should work universally, but testing on OSX would be appreciated (I don't expect anything to break, but dyncall's [README-Developer.txt](https://raw.github.com/perl6/nqp/master/3rdparty/dyncall/dyncall/README-Developer.txt) mentions that OSX uses a custom version of GNU as).

The build semantics are slightly different as the assembler is now always invoked through the gcc frontend (which should not change anything).