gdb/riscv: Fix assertion in inferior call code

gdb/riscv: Fix assertion in inferior call code

An assertion when setting up arguments for an inferior call checks the
size of the argument against xlen.  However, if xlen and flen are
different sizes, and the argument is being placed into a floating
pointer register then we should be comparing against flen not xlen.

This issue shows up as an assertion failure when running on an rv32g
target with a binary compiled using the rv32f abi and making an
inferior call involving large floating point arguments, for example
the test gdb.base/infcall-nested-structs.exp.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_is_fp_regno_p): New function.
	(riscv_register_reggroup_p): Use new function, remove unneeded
	parenthesis.
	(riscv_push_dummy_call): Extend assert to compare against xlen or
	flen based on register type.