x86: update .gitignore entries

vdso / vsycall create .so.dbg files now.
Add *.so.dbg to the main .ignore file

Exclude the compile time created boot directory in arch/x86_64 as well

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>