Workaround 'ELF_DATA/EM_ALPHA redefined' warning in Android linker.h

* dyn_load.c: Include asm/elf.h and linux/elf-em.h explicitly, and
undefine ELF_DATA and EM_ALPHA (if PLATFORM_ANDROID and
BIONIC_ELFDATA_REDEF_BUG are defined) to workaround a bug (avoiding
"macro redefined" warnings) in Android 4.1 (and 4.2) Bionic which has
the mismatching macro definitions in headers included from linker.h.