gdb/riscv: Improve flen length determination

gdb/riscv: Improve flen length determination

This solves an assertion failure when a remote provides a target
description which only refers to floating point registers by their
hardware name (e.g. f0), rather than their ABI name (e.g. ft0). GDB
assumed that should the floating point register feature be presented,
it would contain a register called ft0.

The floating point length is now instead determined by searching for
the same register, but looking for any of its aliases.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_gdbarch_init): Support determining flen from
	target descriptions using exclusively floating point register name
	aliases.