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