x86: clean up after: move entry_64.S register saving out of the macros, fix

Impact: build fix

The break builds with older binutils (2.16.1):

 arch/x86/kernel/entry_64.S: Assembler messages:
 arch/x86/kernel/entry_64.S:282: Error: too many positional arguments
 arch/x86/kernel/entry_64.S:283: Error: too many positional arguments
 arch/x86/kernel/entry_64.S:284: Error: too many positional arguments
 arch/x86/kernel/entry_64.S:285: Error: too many positional arguments
 arch/x86/kernel/entry_64.S:286: Error: too many positional arguments
 arch/x86/kernel/entry_64.S:287: Error: too many positional arguments
 arch/x86/kernel/entry_64.S:288: Error: too many positional arguments
 arch/x86/kernel/entry_64.S:289: Error: too many positional arguments
 arch/x86/kernel/entry_64.S:290: Error: too many positional arguments

Took some time to figure out the detail that GAS chokes on: it's
negative offsets. Rearrange the calculations to make sure we never
go negative.

Signed-off-by: Ingo Molnar <mingo@elte.hu>