[2.0.x] Small assorted collection of fixes and improvements (#10911)

* Misc fixes and improvements

- Get rid of most critical sections on the Serial port drivers for AVR and DUE. Proper usage of FIFOs should allow interrupts to stay enabled without harm to queuing and dequeuing.
  Also, with 8-bit indices (for AVR) and up to 32-bit indices (for ARM), there is no need to protect reads and writes to those indices.
- Simplify the XON/XOFF logic quite a bit. Much cleaner now (both for AVR and ARM)
- Prevent a race condition (edge case) that could happen when estimating the proper value for the stepper timer (by reading it) and writing the calculated value for the time to the next ISR by disabling interrupts in those critical and small sections of the code - The problem could lead to lost steps.
- Fix dual endstops not properly homing bug (maybe).

* Set position immediately when possible

Marlin/src/HAL/HAL_AVR/HAL.h

@@ -64,7 +64,9 @@
   #define CRITICAL_SECTION_START  unsigned char _sreg = SREG; cli();
   #define CRITICAL_SECTION_END    SREG = _sreg;
 #endif
-
+#define ISRS_ENABLED() TEST(SREG, SREG_I)
+#define ENABLE_ISRS()  sei()
+#define DISABLE_ISRS() cli()
 
 // On AVR this is in math.h?
 //#define square(x) ((x)*(x))
@@ -181,7 +183,6 @@ void TIMER1_COMPA_vect (void) { \
     A("lds r16, %[timsk1]")            /* 2 Load into R0 the stepper timer Interrupt mask register [TIMSK1] */ \
     A("andi r16,~%[msk1]")             /* 1 Disable the stepper ISR */ \
     A("sts %[timsk1], r16")            /* 2 And set the new value */ \
-    A("sei")                           /* 1 Enable global interrupts - stepper and temperature ISRs are disabled, so no risk of reentry or being preempted by the temperature ISR */    \
     A("push r16")                      /* 2 Save TIMSK1 into stack */ \
     A("in r16, 0x3B")                  /* 1 Get RAMPZ register */ \
     A("push r16")                      /* 2 Save RAMPZ into stack */ \
@@ -291,7 +292,7 @@ void TIMER0_COMPB_vect (void) { \
     A("out 0x3B, r16")                  /* 1 Restore RAMPZ register to its original value */ \
     A("pop r16")                        /* 2 Get the original TIMSK0 value but with temperature ISR disabled */ \
     A("ori r16,%[msk0]")                /* 1 Enable temperature ISR */ \
-    A("cli")                            /* 1 Disable global interrupts - We must do this, as we will reenable the temperature ISR, and we don´t want to reenter this handler until the current one is done */ \
+    A("cli")                            /* 1 Disable global interrupts - We must do this, as we will reenable the temperature ISR, and we don't want to reenter this handler until the current one is done */ \
     A("sts %[timsk0], r16")             /* 2 And restore the old value */ \
     A("pop r16")                        /* 2 Get the old SREG */ \
     A("out __SREG__, r16")              /* 1 And restore the SREG value */ \