tie up standards macros loose ends (re: 289dd46c)

src/lib/libast/features/standards:
- Do not emit #defines for the typ u_long test which is only used
  as a heuristic in subsequent tests in this file. (Note that 'set'
  can set and unset any iffe command-line --option at runtime.)
- Remove definition of _ISOC99_SOURCE macro. This is another old
  GNU thing; feature_test_macros(7) says invoking the compiler with
  the option -std=c99 has the same effect. But modern GCC has C11
  with GNU extensions as the default, which is fine. If a
  particular standard is desired, pass a -std=... flag in $CC.

src/cmd/ksh93/features/rlimits:
- Remove overlooked Linux *64* types/functions hackery.
  After defining standards macros it caused a build failure
  on at least one version of Void Linux (but not 5.15.14_1).
  Thanks to @JohnoKing for the report.

src/cmd/ksh93/sh/subshell.c,
src/lib/libdll/dllnext.c:
- Remove now-redundant local definitions of _GNU_SOURCE and
  __EXTENSIONS__ macros.

src/cmd/ksh93/tests/builtins.sh:
- Fix broken sed invocation (re: 41829efa).

src/cmd/ksh93/features/rlimits

@@ -1,25 +1,11 @@
 hdr,sys	resource,vlimit sys/time.h
-lib	getrlimit,getrlimit64,ulimit,vlimit
-typ	rlim_t,rlim64_t sys/types.h sys/time.h sys/resource.h
+lib	getrlimit,ulimit,vlimit
+typ	rlim_t sys/types.h sys/time.h sys/resource.h
 
 cat{
 
 #if _sys_resource
 #	include <sys/resource.h>
-#	if _lib_getrlimit64 && _typ_rlim64_t
-#		undef	getrlimit
-#		define	getrlimit	getrlimit64
-#		undef	setrlimit
-#		define	setrlimit	setrlimit64
-#		undef	rlimit
-#		define	rlimit	rlimit64
-#		undef	rlim_t
-#		define	rlim_t	rlim64_t
-#		ifdef RLIM64_INFINITY
-#			undef	RLIM_INFINITY
-#			define	RLIM_INFINITY	RLIM64_INFINITY
-#		endif
-#	endif
 #endif
 
 }end