py: clean up calls to gc_alloc(), reorder setting of FTB, use mp_obj_malloc() to allocate array object - #19707

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (5b5f06c) to head (b4263a7).
⚠️ Report is 5 commits behind head on master.

@@            Coverage Diff             @@
##           master   #19707      +/-   ##
==========================================
+ Coverage   98.55%   98.59%   +0.03%     
==========================================
  Files         182      182              
  Lines       23335    23344       +9     
  Branches        5        5              
==========================================
+ Hits        22998    23015      +17     
+ Misses        336      328       -8     
  Partials        1        1              

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Code size report:

Reference:  mpy-cross/Makefile: Let mpy-cross build in EIM environments. [5b5f06c]
Comparison: unix/coverage: Add coverage test for realloc of GC block with finaliser. [merge of b4263a7]
  mpy-cross:   -16 -0.004% 
   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +8 +0.001% standard
      stm32:   -16 -0.004% PYBV10
      esp32:    +0 +0.000% ESP32_GENERIC
     mimxrt:    -8 -0.002% TEENSY40
        rp2:   -16 -0.002% RPI_PICO_W
       samd:    -8 -0.003% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:   -10 -0.002% VIRT_RV32

I just realised that the CI coverage test here is telling the truth: coverage decreased due to this PR. That's because there's no test for a relocation of a GC block that has the finaliser bit set (that feature is never used by the codebase).

I've now added a coverage test for that, including a new function gc_meta_bits(ptr) which returns the ATB/FTB/WTB bits for the given ptr. I think it's worth adding such a function because it helps a lot to test things are working correctly. It's not guarded by any #if but shouldn't be included in builds that don't use it due to gc-sections.