x86: Properly set MINIMUM_X86_ISA_LEVEL for i386 [BZ #31867]

On i386, set the default minimum ISA level to 0, not 1 (baseline which
includes SSE2).  There are no changes in config.h nor in config.make on
x86-64.  This fixes BZ #31867.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Tested-by: Ian Jordan <immoloism@gmail.com>
Reviewed-by: Sam James <sam@gentoo.org>
Reviewed-by: Florian Weimer <fweimer@redhat.com>