Part-revert GMT->UTC change (re: c9634e9)

To get ksh to prefer UTC over GMT in 'printf %T' output, only the
change in format[] was needed. The corresponding change in zone[]
made it prefer UTC for London time, even in summer time, which is
wrong -- e.g.:

$ LANG=C TZ=Europe/London arch/*/bin/ksh -c 'date; printf %T\\n now'
Tue Jun 30 01:39:09 BST 2020
Tue Jun 30 00:39:09 UTC 2020

src/lib/libast/tm/tmdata.c:
- Revert change in zone[].

src/lib/libast/tm/tmdata.c

@@ -233,10 +233,10 @@ static Tm_leap_t	leap[] =
 
 static Tm_zone_t	zone[] =
 {
- 0,	"UTC",	0,	 ( 0 * 60),	     0,	/* UTC			*/
+ 0,	"GMT",	0,	 ( 0 * 60),	     0,	/* UTC			*/
  0,	"UCT",	0,	 ( 0 * 60),	     0,	/* UTC			*/
+ 0,	"UTC",	0,	 ( 0 * 60),	     0,	/* UTC			*/
  0,	"CUT",	0,	 ( 0 * 60),	     0,	/* UTC			*/
- 0,	"GMT",	0,	 ( 0 * 60),	     0,	/* UTC			*/
  0,	"Z",	0,	 ( 0 * 60),	     0,	/* UTC			*/
  "USA",	"HST",	0,	 (10 * 60),	     0,	/* Hawaii		*/
  0,	"YST",	"YDT",	 ( 9 * 60),	TM_DST,	/* Yukon		*/