targets: add x86-legacy subtarget

As x86-generic is compiled to pentium4 (and newer) there is a need for
a subtarget for older devices. The x86-legacy subtarget is set to
compile to pentium (and newer) and should support even very old devics.

x86-legacy includes the packages from x86.inc to only maintain one
package list. The three excludes are because the packages do not exist
for x86-legacy.