linker: disallow W + E PT_LOAD segments

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

Bug: http://b/30146890
Test: run bionic-unit-tests --gtest_filter=dlfcn.*
Change-Id: Ia23acbe5a48780b65d7e4a50bbe024cd528079f4