bpo-46602: Do not append conftest.c (GH-31062)

The heredoc creation statements use >> to append conftest.c.  This can cause
tricky build issues if the file is not correctly removed prior to its
name being reused (such name is reused several times for different
contextual tests during the build).  One such result from appending may
cause #include <ac_nonexistent.h> to persist when testing to acquire
PLATFORM_TRIPLET.  This can then lead to downstream issues concerning SOABI.