lib: libc: newlib: Initialise libc heap during POST_KERNEL phase

This commit changes the invocation of the newlib malloc heap
initialisation function such that it is executed during the POST_KERNEL
phase instead of the APPLICATION phase.

This is necessary in order to ensure that the application
initialisation functions (i.e. the functions called during the
APPLICATIION phase) can make use of the libc heap.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>