drop GDB_LIBEXEC option (now always true)

In-tree gdb is essentially obsolete. We kept it for sparc64 (because
gdb in ports lacked sparc64 support) and as a fallback for crashinfo.
gdb was installed to /libexec on all archs other than sparc64, where the
WITHOUT_GDB_LIBEXEC option was default, with gdb installed to /usr/bin.
With sparc64's retirement WITH_GDB_LIBEXEC became the default for all
architectures, but it was still possible to set it off and install gdb
into /usr/bin.

As the next step in gdb's retirement, remove the option and install gdb
only into /libexec as the crashinfo fallback. We expect users to install
the gdb port or package for debugging. The in-tree gdb lacks support for
a number of supported architectures and does not support contemporary
DWARF debug info.

Reviewed by: jhb (earlier)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24227