Skip check for unportable sample pack/unpack when cross-compiling
memaccess.cmake: Cosmetics, whitespace, consistency fixes.

Match the rest of PulseView's CMake file conventions more closely.
memaccess.cmake: Use STATUS instead of WARNING for output.

Using STATUS looks nicer here (and it's an informational message only).

Old:
CMake Warning at CMake/memaccess.cmake:39 (MESSAGE):
  Cross compiling - using portable code for memory access
Call Stack (most recent call first):
  CMakeLists.txt:93 (memaccess_check_unaligned_le)

New:
-- Cross compiling - using portable code for memory access