cleanup kernel flags.

-mno-mmx implies -mno-3dnow and -mno-3dnowa
-mno-sse implies -mno-sse2, -mno-sse3, -mno-ssse3, -mno-sse41 and -mno-sse42

add -mno-aes and -fno-strict-overflow

from freebsd
only add -no-integrated-as to the files clang can't compile itself.
remove -fno-strict-overflow
remove unneeded .end directive.
Add -mno-omit-leaf-frame-pointer, which is not implied by clang when
using -fno-omit-frame-pointer
prepend __ to asm and volatile
Check that ksh array indices are <= INT_MAX to prevent integer overflow.

pointed out by Todd C. Miller
agreed to by oga@