Enable UBSAN_BOUNDS and UBSAN_SHIFT

Ubuntu, Android, and Fedora have had UBSAN_BOUNDS enabled for a couple years now, and real flaws are being found. UBSAN_SHIFT appears to have had all the corner cases shaken out via syzkaller over the last several years.

This produces warnings only so anyone wanting to enforce the bounds checking needs to set the warn_limit (or panic_on_warn) sysctl. Image growth for this is under 5% in the worst case (largely due to the growth of .rodata for the warning strings).