Improve Teensy 4.x support, add hardware PWM for Laser / Spindle / FAST_PWM_FAN

[phizev]

Description

This is an attempt at providing more complete support for Teensy 4.x boards, this includes:

• Additional sanity checks.
• Working freeMemory() function.
• Hardware PWM / FAST_PWM_FAN / SPINDLE_FEATURE / LASER_FEATURE support.

Requirements

• Teensy 4.x based board.

Benefits

• Allows use of SPINDLE_FEATURE / LASER_FEATURE.
• Allows use of FAST_PWM_FAN.
• Working freeMemory() function.
• Increased build time checks.

Configurations

#define MOTHERBOARD BOARD_TEENSY41

For checks:

• #define SERIAL_STATS_MAX_RX_QUEUED (Or other SERIAL_STATS_*)
• #define M100_FREE_MEMORY_WATCHER
• #define BAUD_RATE_GCODE and #define SERIAL_PORT_2 -2

For hardware PWM:

• #define FAST_PWM_FAN

Laser / Spindle:

• #define SPINDLE_FEATURE
• #define LASER_FEATURE
• #define SPINDLE_LASER_USE_PWM

Working freeMemory() function

• Just check free memory on serial during printer start.

Related Issues

[FR] LASER / SPINDLE pwm control on teensy 4.1

Notes on freeMemory() and Teensy 4.x memory layout

Regarding freeMemory(), it reports memory free for it's second RAM region used for malloc(), from https://forum.pjrc.com/index.php?threads/how-to-display-free-ram.33443/#post-275506

... the code in msg #6 will tell you the portion of RAM2 which is still unused by malloc() & C++ new.

One way Teensy 4.x differs from earlier Teensy and many other microcontrollers is the local variables are in a physically different memory than the heap for malloc().

(More recent versions of the Platform.io teensy platform make this arrangement clearer in the build summary, though updating that is going to be a follow up PR if I find a workable solution for changes to the SD library dependency other than just including the old library via #lib_deps.)