net: lwm2m: oma_tlv: Fix UBSAN warnings about signed integers

* Simplify the logic of the get_number() function to address the corner
  cases reported by UBSAN regarding byte-swapping signed integer values.

  The existing logic was actually only valid for little-endian systems,
  as it expected that the bytes written from the packet buffer will be
  stored at the beginning of the int64_t memory, plus the actual
  byte-swapping with signed integer casts inside was hard to follow.
  Switch to a plain uint8_t buffer for integer readout, and use
  dedicated system function to convert the big-endian data in the buffer
  into unsigned integer in the system endianness, followed by the final
  cast to a signed value.

* Add explicit cast to uint32_t in put_objlnk() to prevent warning about
  not-fitting integer after byte shift, and update the result type to
  uint32_t as well.

* Switch to buffer with sys_put_be16/32/64 when writing integers due to
  similar warnings about byte-swapping signed values.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>