gitignore: x86_64 files

Add filters for x86_64 generated files.
Signed-off-by: NBrian Gerst <bgerst@didntduck.org>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

arch/x86_64/boot/.gitignore

+bootsect
+bzImage
+setup

arch/x86_64/boot/tools/.gitignore

+build

arch/x86_64/ia32/.gitignore

+vsyscall*.so