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

gnu/usr.bin/gdb/Makefile

@@ -4,10 +4,4 @@
 
 SUBDIR=	libgdb gdb kgdb
 
-.if ${MK_GDB_LIBEXEC} == "no"
-.if exists(${.CURDIR}/gdbserver/reg-${MACHINE_CPUARCH}.c)
-SUBDIR+=gdbserver
-.endif
-.endif
-
 .include <bsd.subdir.mk>

gnu/usr.bin/gdb/Makefile.inc

@@ -58,7 +58,7 @@ GENSRCS+= nm.h tm.h
 CFLAGS+= -DCROSS_DEBUGGER -I${BMAKE_ROOT:H:H}
 GDB_SUFFIX= -${TARGET_ARCH}
 MAN=
-.elif ${MK_GDB_LIBEXEC} != "no"
+.else
 BINDIR?=	/usr/libexec
 MAN=
 .endif