Not wise if stddef.h was already included. This is related to commit
3aa26a794e
Instead hack stddef.h to have identical definition and thus
avoid the issue mentionned there.
- add __builtin_va_arg_types to check how arguments were passed
- move most code of stdarg into libtcc1.c
- remove __builtin_malloc and __builtin_free
- add a test case based on the bug report
(http://www.mail-archive.com/tinycc-devel@nongnu.org/msg03036.html)
(Because GNU's alloca.h unconditionally #undef's alloca)
Also, remove gcc specific sections in headers. and
instead change tests such that gcc does not use them.
