gdb: Update type of lower bound in value_subscripted_rvalue

gdb: Update type of lower bound in value_subscripted_rvalue

The dynamic lower (and upper) bounds of ranges are stored as type
LONGEST (see union dynamic_prop_data in gdbtypes.h).  In most places
that range bounds are handled they are held in a LONGEST, however in
value_subscripted_rvalue the bound is placed into an int.

This commit changes value_subscripted_rvalue to use LONGEST, there
should be no user visible changes after this commit.

gdb/ChangeLog:

	* valarith.c (value_subscripted_rvalue): Change lowerbound
	parameter type from int to LONGEST.
	* value.h (value_subscripted_rvalue): Likewise in declaration.