Fix build of stand with base gcc

Fix build of stand with base gcc

* Make autoboot() a static function in stand/common/boot.c, so it does
  not shadow local variables in gptboot.c and zfsboot.c.
* Remove -Winline from the Makefiles for gptboot, gptzfsboot and
  zfsboot, as gcc will always fail to inline some functions, and there
  is nothing we can do about it.
* For gcc <= 4.2.1, silence -Wuninitialized for isoboot, as it produces
  a false positive warning.
* Remove deprecated and unnecessary -mcpu=i386 flag from stand/defs.mk,
  as there is already a -march=i386 flag further in the file.

Reviewed by:	imp
MFC after:	3 days
Differential Revision: https://reviews.freebsd.org/D15628