py/gc: Reduce code size when MICROPY_GC_SPLIT_HEAP is disabled.

Use C macros to reduce the size of firmware images when the GC split-heap
feature is disabled.

The code size difference of this commit versus HEAD~2 (ie the commit prior
to MICROPY_GC_SPLIT_HEAP being introduced) when split-heap is disabled is:

       bare-arm:    +0 +0.000%
    minimal x86:    +0 +0.000%
       unix x64:   -16 -0.003%
    unix nanbox:   -20 -0.004%
          stm32:    -8 -0.002% PYBV10
         cc3200:    +0 +0.000%
        esp8266:    +8 +0.001% GENERIC
          esp32:    +0 +0.000% GENERIC
            nrf:   -20 -0.011% pca10040
            rp2:    +0 +0.000% PICO
           samd:    -4 -0.003% ADAFRUIT_ITSYBITSY_M4_EXPRESS

The code size difference of this commit versus HEAD~2 split-heap is enabled
with MICROPY_GC_MULTIHEAP=1 (but no extra code to add more heaps):

    unix x64: +1032 +0.197% [incl +544(bss)]
       esp32:  +592 +0.039% GENERIC[incl +16(data) +264(bss)]

0 files changed, 0 insertions, 0 deletions