linker: warn about W+E load segments

No mapped segment from the elf file can be writable and
executable at the same time. This commit adds a warning
for malformed PT_LOAD segments in the elf-files.

Change-Id: I5e1e41bc2aabc209d3b07dd6ca2689a394294628
Test: flip protection flag for libtest_empty.so (used hexedit), adb sync
Test: run bionic-unit-tests and make sure it displays the warning.
Bug: http://b/30146890