uapi/linux/a.out.h: don't use deprecated system-specific predefines.

uapi/linux/a.out.h uses a number of predefined macros that are
deprecated because they're in the application namespace
(e.g. '#ifdef linux' instead of '#ifdef __linux__').
This patch either corrects or just removes them if they are not
applicable to Linux.

The primary reason this is worth bothering to fix, considering how
obsolete a.out binary support is, is that the GCC build process
considers this such a severe error that it will copy the header into a
private directory and change the macro names, which causes future
updates to the header to be masked.  This header probably doesn't get
updated very often anymore, but it is the _only_ uapi header that gets
this treatment, so IMHO it is worth patching just to drive that number
all the way to zero.

Signed-off-by: Zack Weinberg <zackw@panix.com>
[hch: removed dead conditionals]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>