x86: fix xsave build error

fix this build failure with certain glibc versions:

In file included from /usr/include/bits/sigcontext.h:28,
                 from /usr/include/signal.h:333,
                 from Documentation/accounting/getdelays.c:24:
/home/mingo/tip/usr/include/asm/sigcontext.h:191: error: expected specifier-qualifier-list before ‘u64’

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