typedef int128_t int128_t_a8 __attribute__ ((aligned(8)));

Struck the same issue messing with the __float128 type (Math-Float128) ... at least, the solution you provided worked for me, too.

What I find interesting is that there is a need to apply this measure only when using the 64-bit MinGW compiler. With the 32-bit MinGW compilers, there's no need.
Mind you, Strawberry Perl's gcc-4.6.3 compilers (both 32-bit and 64-bit) create runtime crashes whenever quadmath.h's expq() function is called - I couldn't reproduce that problem anywhere else.

Anyway, salva, I'm glad you found an answer to the problem - I hate to think how long it might have taken me to find this solution (let alone decipher what the problem was in the first place).

Cheers,
Rob