Sanitise standards/feature macros, remove compiler/linker wrappers

The goal is to get rid of all compiler/linker wrapper scripts as
they are overridden by passing CC/LD and it should be possible to
select your compiler or linker without breaking the build. The
probing and feature testing system should set the appropriate flags
and macros. This makes some progress towards that.

src/lib/libast/features/standards:
- Eliminate the shotgun approach to standards macros on popular
  systems where the macros we we need to set are known and
  documented. The following will enable standards compliance plus
  all the available extensions:
  - Set no macros at all for any BSD system (excluding macOS).
  - Set _DARWIN_C_SOURCE on Darwin/macOS.
  - Set everything and the kitchen sink for Solaris/illumos in
    a way that enables backwards compatibility with older Solaris.
    This is unofficial, but following the standards(5) manual
    disables a lot of basic functionality that we depend on.
  - Set _GNU_SOURCE for GNU (glibc).
  - Remove the covered macros from the shotgun approach fallback.
- Add a new heuristic. _POSIX_PATH_MAX and _SC_PAGESIZE are among
  the basic macros disabled when you pass recommended standards
  macros, killing the build, so it's good to check if they compile.

src/cmd/INIT/ar.freebsd12.amd64,
src/cmd/INIT/ar.linux.i386-64:
- Removed. May cause build failures on some systems as not all 'ar'
  implementations support the U option. Plus, I can think of no
  good reason to disable deterministic mode (which always creates
  identical output) on 'ar' implementations that support it. See:
  https://groups.google.com/g/comp.unix.shell/c/LdOD1Ya0C9E/m/U6DhgHVICwAJ

src/cmd/INIT/cc.linux.*-icc,
  Removed icc wrappers. These manually source /etc/profile.d/icc.sh
  but I don't think that is the build system's job. Profile scripts
  should be run at login time and export variables we inherit
  through the environment.

src/cmd/INIT.cc.{freebsd,linux,openbsd}*:
- Removed. Should be entirely superfluous now that the standards
  feature test sets the appropriate macros.

src/cmd/INIT.cc.sol11.*:
- Removed as the standards feature test now sets the approopriate
  macros. Note the Solaris build system should now simply pass CC
  as normal instead of passing CC_EXPLICIT.

Author: McDutchie