[gdb] Fix grammar in comments and docs

Fix grammar in some comments and docs:
- machines that doesn't -> machines that don't
- its a -> it's a
- its the -> it's the
- if does its not -> if it does it's not
- one more instructions if doesn't match ->
  one more instruction if it doesn't match
- it's own -> its own
- it's first -> its first
- it's pointer -> its pointer

I also came across "it's performance" in gdb/stubs/*-stub.c in the HP public
domain notice, I've left that alone.

Tested on x86_64-linux.

gdb/amd64-tdep.c

@@ -1750,7 +1750,7 @@ amd64_displaced_step_fixup (struct gdbarch *gdbarch,
 	 system calls.	*/
       if (amd64_syscall_p (insn_details, &insn_len)
 	  /* GDB can get control back after the insn after the syscall.
-	     Presumably this is a kernel bug.  Fixup ensures its a nop, we
+	     Presumably this is a kernel bug.  Fixup ensures it's a nop, we
 	     add one to the length for it.  */
 	  && (pc < to || pc > (to + insn_len + 1)))
 	displaced_debug_printf ("syscall changed %%rip; not relocating");