libm: Add several new functions and symbol versioning

The following long double functions were added to the math library:
  logl
  log2l
  log10l
  log1pl
  expm1l
  acoshl
  asinhl
  atanhl

In addition, the FreeBSD functionality that creates symbol versioning
for libraries was adapted for FreeBSD.  The first version is called
"DFLY36.0".  If it is necessary to create a new version of the 3.5 or
3.6 branch, the number after the decimal will be incremented.  The 3.7
branch will start with "DFLY38.0" if it needs its own version.

libm was baselined with all symbols being the same version: DFLY36.0.
With symbol versioning, it will not be necessary to increment the major
version anymore, so this library shall always be known as libm.so.4 from
this point on.
Change initial symbol version from DFLY36.0 to DF306.0

The first one will make handling DragonFly 3.9 and later awkward.
Remove MACHINE_ARCH=amd64 and legacy make instructions from makefiles

Apparently the x86_64 platform used to be referred to as "amd64" so some
makefile code was added to help with the transition.  It probably should
have been removed when bringing bmake in.

With the commit, the makefiles expect bmake and will break if an older
make is used (e.g. from DragonFly 3.2).  That means an upgrade to
DragonFly 3.6 will have to be upgraded to DragonFly 3.4 first.  There
may be other reasons to do this as well, besides just bmake.

The recent symbol versioning makefile for libraries requires bmake, so
legacy make can't build world anymore in any case.
Merge branch 'master' into gsocdrm_jfree