Use standard pattern for stdargs

[imp]

Summary

mips stdargs.h has all kinds of junk for really old compilers. gcc 3 was released in 2001, so we don't need support for pre-gcc3 anymore. When you remove it, there's nothing left except for code that's a poorer version of sys/_stdarg.h. Migrate to that. In fact, sparc64 and powerpc could use the same.

Test Plan

I'm just doing a buildworld :) worked for mipsel, mips64, powerpc64, sparc64

[imp]

I'll note that phk suggests that mips can't use this file in his initial commit, but I see no reason why it can't.

[imp]

buildworld mips64 and mipsel completes w/o error

[imp]

sparc and powerpc too

[imp]

• We don't support gcc < 4.2.1, so varargs.h now is just #error

[kan]

no objection

[jhibbits]

If it builds, ship it.