This makes available the __builtin_va_list type and __builtin variants
of va_start, va_arg, va_copy and va_end. We do this via a header file
that's prepended to all compilations always (except if merely
preprocessing): tcc_predefs.h. That header could also be used
for predefining other builtins in the future.
We don't need the define hacks for musl anymore with this.
Also fix x86_64 gfunc_prologue to reserve enoug space for the
full va_list structure, not just 16 bytes.
