[gnumach] Support GCC5

[David Michael]

Hi,

Can this file be added to gnumach's linux/src/include/linux directory?

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/include/linux/compiler-gcc5.h

And just a heads-up:  With -std=gnu11 by default, there are two errors
due to asm input/output operands being in the clobber list, and
linking fails horrifically due to the difference in "extern inline"
behavior.  Adding -std=gnu89 to CFLAGS works around this.

Thanks.

David