Without having __riscv_flen defined we get multiple warinings. But
defining it causes the startup code to contain floating point instructions.
This results in a exception right after booting.

See startup_gd32vf103.S lines 289-294 should open a PR at nuclei sdk

This reverts commit 5e0c2e1.

The core of tinyusb must be as independent as possible, we previously
relied on nuclei-sdk or the GD32VF103 firmware library for the synopsys
driver to work with the GD32VF103. Fortunatly we needed very few parts
from them so we implement them here.

This controller family only supports USB FS with four endpoints

We use the linker files provided by nuclei-sdk instead

Instead of using the HAL functions we can just use the defines from the
board support for the longan nano that comes with the nuclei-sdk. Also
we move some includes and defines to the header file.

Only 48MHz, 72MHz, 96MHz and 120 MHz system clocks derived from an
external crystal are suitable for the usb peripheral, as the internal
oscillator is not stable enough. Also the usb-prescaler only supports
division by 1 (48MHZ), 1.5(72MHz), 2(96MHz) and 2.5(120Mhz).
120Mhz is also out of spec and not added here.

Forgot to reload the systick timer in the irq handler

The GD32VF103 family only has USB-OTG peripherals.

As the peripheral is the same as on the STM32F1 and STM32F4 lines we do the same.

The systick timer is driven by the AHB bus divided by 4, set the correct
reload value to generate a timer irq every ms.