But check that mkfs.c is building with LP64

Search for an addr2line that supports elf32-i386.  Don't print the
path of addr2line as a side-effect of which.  Don't uselessly pipe
"grep '^ '" to addr2line's stdin.  Enable as many pretty-printing and
otherwise helpful options as possible (this makes the output *much*
more pleasant on modern addr2lines).

0 is not a system call (thanks to Peter Froehlich)

Previously, these were inconsistent: they used their struct proc
argument for bounds checking, but always copied the argument from the
current address space (and hence the current process).  Drop the
struct proc argument and always use the current proc.

Suggested by Carmi Merimovich.