Teach libstdc++ about logl(3).

The logl(3) family of functions were implemented in r251292.
Define them in libstdc++'s configuration so they can be used.

Reviewed by:	dim
kdump: Decode AT_FDCWD in first argument of bindat() and connectat().
vfork(2) was listed as deprecated in 1994 (r1573) and was the false
reports of its impending demise were removed in 2009 (r199257).

However, in 1996 (r16117) system(3) was switched from vfork(2) to
fork(2) based partly on this.  Switch back to vfork(2).  This has a
dramatic effect in cases of extreme mmap use - such as excessive
abuse (500+) of shared libraries.

popen(3) has used vfork(2) for a while.  vfork(2) isn't going anywhere.
Remove get_rev_branch(), functionality exists in the release/Makefile.

Submitted by:	hrs
Rename imx_machdep.c to imx51_machdep.c, because it contains hardware
addresses which are specific to the imx51 chips.