x86: Fix breakage of UML from the changes in the rwsem system

x86: Fix breakage of UML from the changes in the rwsem system

The patches 5d0b7235d83eefdafda300656e97d368afcafc9a and
bafaecd11df15ad5b1e598adc7736afcd38ee13d broke the UML build:

On Sun, 17 Jan 2010, Ingo Molnar wrote:
>
> FYI, -tip testing found that these changes break the UML build:
>
> kernel/built-in.o: In function `__up_read':
> /home/mingo/tip/arch/x86/include/asm/rwsem.h:192: undefined reference to `call_rwsem_wake'
> kernel/built-in.o: In function `__up_write':
> /home/mingo/tip/arch/x86/include/asm/rwsem.h:210: undefined reference to `call_rwsem_wake'
> kernel/built-in.o: In function `__downgrade_write':
> /home/mingo/tip/arch/x86/include/asm/rwsem.h:228: undefined reference to `call_rwsem_downgrade_wake'
> kernel/built-in.o: In function `__down_read':
> /home/mingo/tip/arch/x86/include/asm/rwsem.h:112: undefined reference to `call_rwsem_down_read_failed'
> kernel/built-in.o: In function `__down_write_nested':
> /home/mingo/tip/arch/x86/include/asm/rwsem.h:154: undefined reference to `call_rwsem_down_write_failed'
> collect2: ld returned 1 exit status

Add lib/rwsem_64.o to the UML subarch objects to fix.

LKML-Reference: <alpine.LFD.2.00.1001171023440.13231@localhost.localdomain>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>