elf: Add a tunable to control use of tagged memory

Add a new glibc tunable: mem.tagging.  This is a decimal constant in
the range 0-255 but used as a bit-field.

Bit 0 enables use of tagged memory in the malloc family of functions.
Bit 1 enables precise faulting of tag failure on platforms where this
can be controlled.
Other bits are currently unused, but if set will cause memory tag
checking for the current process to be enabled in the kernel.