shells/ast-ksh: fix build on powerpc64*

Bring back patches committed as part of r346423 to fix build issue:
/wrkdirs/usr/ports/shells/ast-ksh/work/ast-ksh93v/src/lib/libast/hash/hashalloc.c:162:4: error: non-const lvalue reference to type '__builtin_va_list' cannot bind to a temporary of type 'va_list' (aka 'char *')

va_copy(ap, va_listval(va_arg(ap, va_listarg)));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/include/sys/_stdarg.h:49:32: note: expanded from macro 'va_copy'

#define     va_copy(dest, src)      __va_copy(dest, src)
                                    ^~~~~~~~~~~~~~~~~~~~

/usr/include/sys/_stdarg.h:47:58: note: expanded from macro '__va_copy'

#define       __va_copy(dest, src)    __builtin_va_copy((dest), (src))
                                                                ^~~~~

1 error generated.

PR: 255308
Approved by: saper@saper.info (maintainer)