[SjLj] Don't use __declspec(thread) in MinGW mode

GCC and Clang in MinGW mode don't support __declspec(thread), which
after expanding macros ends up as __attribute__((thread)). Use the
GCC specific attribute __thread instead (the next one in the chain
of alternatives).

Differential Revision: https://reviews.llvm.org/D56905

llvm-svn: 351587