lib: libc: newlib: define USED_RAM_END_ADDR only where it is needed

USED_RAM_END_ADDR is not used when Kconfig option
CONFIG_NEWLIB_LIBC_ALIGNED_HEAP_SIZE is defined,
therefore, we do not need to define the macro in
that specific case.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>