libsframe: write out SFrame FRE start address correctly

The following test was failing on ppc64 and s390x:
  "FAIL: encode-1: Encode buffer match"

The offending stub was how we memcpy the FRE start address to the buffer
(on-disk format).  When the host is big-endian, the address of the
source buffer for the memcpy needs to point to the uint8_t/uint16_t sized
value of the FRE start addr, not uint32_t sized value; we intend to copy
out only the fre_start_addr_sz number of bytes.

ChangeLog:

	* libsframe/sframe.c (sframe_encoder_write_fre_start_addr): New
	function.
	(sframe_encoder_write_fre): Use it instead of memcpy.