Fix <machine/varargs.h> use cases.

First varargs.h depended on namespace pollution to provide typdef of __va_list
to declare va_list. Usually thorugh sys/systm.h including sys/stdarg.h

So short-cir

Fix <machine/varargs.h> use cases.

First varargs.h depended on namespace pollution to provide typdef of __va_list
to declare va_list. Usually thorugh sys/systm.h including sys/stdarg.h

So short-circuit directly to compiler builtin in case of __GNUC__

Also remove machine/varargs.h usage from other kernel sources:
sys/kern/kern_dsched.c: Not needed (just 3 dummy functions)

sys/dev/misc/tbridge/tbridge.c: Both use just __va_smth variants
sys/kern/subr_taskqueue.c: and get those through sys/systm.h

This leaves all the kernel code using <stdarg.h> variant consistently.

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