Additional lib/libc/arch/ build break on evbppc64

[Paul Goyette <paul%whooppee.com@localhost>]

Christos's recent fix for syncicache.c worked, but now we're broken at

--- _lwp.ln ---
/build/netbsd-local/src/lib/libc/arch/powerpc64/gen/_lwp.c(61): warning: 
possible pointer alignment problem [135]
/build/netbsd-local/src/lib/libc/arch/powerpc64/gen/_lwp.c(61): warning: 
pointer casts may be troublesome [247]
/build/netbsd-local/src/lib/libc/arch/powerpc64/gen/_lwp.c(51): warning: 
argument private unused in function _lwp_makecontext [231]
*** [_lwp.ln] Error code 1


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