xtensa: Fix implicit enum conversions

xtensa: Fix implicit enum conversions

XTREG is used with raw numbers, like:

...
  XTREG( 78,312,32, 4, 4,0x02d7,0x0007,-2, 2,0x1000,excsave7,    0,0,0,0,0,0)
  XTREG( 79,316, 8, 4, 4,0x02e0,0x0007,-2, 2,0x1000,cpenable,    0,0,0,0,0,0)
  XTREG( 80,320,22, 4, 4,0x02e2,0x000b,-2, 2,0x1000,interrupt,   0,0,0,0,0,0)
  XTREG( 81,324,22, 4, 4,0x02e2,0x000d,-2, 2,0x1000,intset,      0,0,0,0,0,0)
...

So just add the explicit cast to the macro.

In file included from /home/pedro/gdb/mygit/src/gdb/xtensa-config.c:25:0:
/home/pedro/gdb/mygit/src/gdb/xtensa-tdep.h:289:2: error: invalid conversion from ‘int’ to ‘call_abi_t’ [-fpermissive]
  }
  ^

gdb/ChangeLog:

	* xtensa-tdep.h (XTREG): Add casts.
	(XTREG_END): Likewise.