build: Require yasm 0.8.0 or higher

This is in preparation for removing a .rodata kludge
which was only required for older YASM versions.

The movbe instruction was introduced in 0.8.0, which already
had AVX, which was introduced in 0.7.0, and NASM introduced
movbe in 2.0.3, which is the same version which introduced
AVX support.

Also, make the failure message more accurate.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
x86inc: Remove .rodata kludges

The Mach-O bug was fixed in yasm 0.8.0 and we don't
support versions that old anymore.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>