[Bug target/115054] __float128 and _Float16 use incorrect ABI on x86-64 MinGW

[tmgross at umich dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054

--- Comment #5 from Trevor Gross <tmgross at umich dot edu> ---
Looks like bugz cut off part of the sysv quote, here for reference:

> Arguments of types __float128, _Decimal128 and __m128 are split
> into two halves. The least significant ones belong to class SSE, the most
> significant one to class SSEUP.