crt: Localize usage of startinfo variable

Variable startinfo is used only by __(w)getmainargs() call passed by the
pointer in pre_cpp_init() function. The __(w)getmainargs() function does
not use or store the pointer value, so the variable does not have to be
static. Declare this variable just on the pre_cpp_init() function stack.

The _startupinfo struct is an input-only struct with one member
(int newmode).

Signed-off-by: Martin Storsjö <martin@martin.st>