Fixes for the umip test

When compiling umip.c with -O2 instead of -O1, there are currently
two problems. First, the compiler complains:

 x86/umip.c: In function ‘do_ring3’:
 x86/umip.c:162:37: error: array subscript 4096 is above array bounds of
    ‘unsigned char[4096]’ [-Werror=array-bounds]
       [user_stack_top]"m"(user_stack[sizeof user_stack]),
                           ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~

This can be fixed by initializing the stack to point to one of the last
bytes of the array instead.

The second problem is that some tests are failing - and this is due
to the fact that the GP_ASM macro uses inline asm without the "volatile"
keyword - so that the compiler reorders this code in certain cases
where it should not. Fix it by adding "volatile" here.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200512094438.17998-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>