simpleperf: fix is_elf_file.

When running by python3, is_elf_file() decodes first 4 bytes of
a file, which may cause UnicodeDecodeError.

Bug: none
Test: run test.py TestTools.test_is_elf_file.
Change-Id: Ie19e4869f876bb92155d849c814c8f39989c443b
2 files changed