Use __va_copy as fallback fo va_copy and a = b as fallback of the fallback.

This is needed because clang in c89 mode implements __va_copy
(it was in draft version of the spec) but forbids assignments of __builtin_va_list).
Merge pull request #11 from cesanta/va_copy_code_review

Use __va_copy as fallback fo va_copy and a = b as fallback of the fallback