crt: tests: update t_fctptr

This test has two issues:

1. typedefs for `malloc` and `free` are missing `__cdecl`; this would result in
  compilation error on i386 as all CRT function have __cdecl calling convention.

2. `malloc` can fail under low memory conditions, resulting in a spurious test
  failure.

Instead of testing `malloc` and `free`, call `exit` through a pointer, which
should exit process with zero. If for any reason call to `exit` did not exit
the process, call `_exit` to exit with non-zero to signal an error.

Add t_fctptr to testcases/Makefile.am.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>