This PR enables building of perl with USE_QUADMATH on MS Windows using (32-bit and 64-bit) mingw-w64 ports of gcc.

Unforunately, having it smoked is probably not going to prove much, and the same can be said of the CI testing that I expect this PR to initiate.
AFAIK there are currently no smokers set up to attempt a USE_QUADMATH build on Windows - the best that can be shown by smoking it is that these changes haven't broken pre-existing functionality.
I've run extensive testing locally which has established (to my satisfaction, at least) that no such "pre-existing functionality" has been altered.
Furthermore, that same extensive testing has shown that the USE_QUADMATH builds themselves are fine.

With both 'gmake' and 'dmake', I have built (with the files contained in this PR) the following configurations of perl:

MSWin32-x86-multi-thread
MSWin32-x86-multi-thread-ld
MSWin32-x86-multi-thread-quadmath
MSWin32-x86-multi-thread-64int
MSWin32-x86-multi-thread-64int-ld
MSWin32-x86-multi-thread-64int-quadmath

MSWin32-x64-multi-thread
MSWin32-x64-multi-thread-ld
MSWin32-x64-multi-thread-quadmath

For each of those (9 x 2 == 18) builds I ran (n|g)make, (n|g)make test, (n|g)make install, and (n|g)make distclean.

Everything was fine, except for the following caveat that applies only to the "MSWin32-x86-multi-thread-quadmath" configuration:
For this build (with both 'gmake' and 'dmake'), I get some Math::BigInt/BigFloat test failures. Those test failures are identical for both flavours of make.
I don't think this should be considered to be a blocker.
Firstly, it's not breaking any existing builds.
Secondly, this configuration has ivsize=4 and nvtype of __float128, which is a very rare configuration. (I doubt that there are any smokers for any system that test such a configuration. Right ?)
I don't see it as a configuration that will be highly favoured. (Far more likely that a selection of ivsize==8 will be desired when nvtype is set to __float128.)
Of course, I'll investigate this strange issue anyway - with an aim to fixing it with a separate PR, after this one has been merged into blead.

Cheers,
Rob