[build.sh] Specify -march for mingw-builds.

Some mingw-builds toolchain by default use -march=i686 by default,
and use SSE instructions, including movdqa on stack which is not
always aligned to 16-byte boundary, leading to crash even on CPUs
having the SSE support.

Hence we explicitly use -march=i586 -mtune=core2 for x86 builds
with mingw-builds.