Get the program name for MTE property check.

The property check in is_permissive_mte() uses getprogname() which
doesn't get the correct program name. Instead use the libbase
GetExecutablePath() function instead.

Add a new test for the fallback path that runs in minijail to verify
that there is no SIGSYS raised.

Add the readlinkat syscall for MTE but only on aarch64.

Test: Verified that the property check actually sees the executable name.
Test: All unit tests pass on an MTE enabled device.
Change-Id: I44276f6cdc4860a88e1f48a2011ca4c4e4f1dc2a