Use <stdarg.h> instead of <machine/stdarg.h> in userland.

<machine/stdarg.h> is a kernel-only header. The standard header for
userland is <stdarg.h>. Using the standard header in userland avoids
weird build errors when building with external compilers that include
their own stdarg.h header.

Reviewed by: arichardson, brooks, imp
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D14776