Add the fnptr C callback test (direct symbol, volatile local pointer and
global pointer), its GSPA entry and marker script, and scripts/tests/m4/
fnptr.sh. The gate compiles at the default -O2 pipeline, requires all four
markers to be 42 in MAME, and checks the linked assembly: _apply calls through
an A register (CALL, never CALLA) after the argument push, while _fnptr_test
keeps the direct CALLA and materialises inc with MOVI. It also pins the GSPA
0x0920 encoding and requires dis34 to decode the call as 'call aN'.
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