Read LINUX_VERSION_CODE from vDSO ELF header instead of uname

[ti-mo]

Continuation of #489 with a few changes:

• moved the _AT_SYSINFO_EHDR tag to the end of the auxv test before _AT_NULL to test unmatched/fallthrough cases (ehdr used to be the first element in the vector)
• read auxv unbuffered without ReadAll() since we don't have any control over its size. Avoids most allocs at the cost of more reads but auxv is already an in-memory structure in practice. Also being a bit more liberal regarding alignment, removing the double-buffering and reading it using an anon struct with Tag and Val fields.
• removed one of the buffers in parsing ELF notes
• using unix.ByteSliceToString for reading nul-terminated strings
• added a test for an auxv blob without a vdso pointer

Thanks @brycekahle!