i386: Add __attribute__ ((gcc_struct)) to struct fenv [PR95418]

Windows ABI (MinGW) is different than Linux ABI when bitfileds are involved.
The following patch adds __attribute__ ((gcc_struct)) to struct fenv in order
to match the layout of x87 state image in memory.

2020-06-01  Uroš Bizjak  <ubizjak@gmail.com>

libatomic/ChangeLog:
	* config/x86/fenv.c (struct fenv): Add __attribute__ ((gcc_struct)).

libgcc/ChangeLog:
	* config/i386/sfp-exceptions.c (struct fenv):
	Add __attribute__ ((gcc_struct)).

libgfortran/ChangeLog:
	PR libfortran/95418
	* config/fpu-387.h (struct fenv): Add __attribute__ ((gcc_struct)).