tcg: improve comment about pure functions

#define TCG_CALL_TYPE_REGPARM_1 0x0001 /* i386 style regparm call (1 reg) */

#define TCG_CALL_TYPE_REGPARM_2 0x0002 /* i386 style regparm call (2 regs) */

#define TCG_CALL_TYPE_REGPARM   0x0003 /* i386 style regparm call (3 regs) */

/* A pure function only reads its arguments and globals variables and

   cannot raise exceptions. Hence a call to a pure function can be

/* A pure function only reads its arguments and TCG globals variables

   and cannot raise exceptions. Hence a call to a pure function can be

   safely suppressed if the return value is not used. */

#define TCG_CALL_PURE           0x0010