#4036 mos6502 crash for test/kyber-ntt.c on big endian host

[Philipp Klaus Krause]

This is a regression. The test passed at least up to [r16688], but SDCC crashes (on big-ednidan hosts) as of [r16695] and [r16696], which is current trunk.

Running uc6502 regression tests
Caught signal 11: SIGSEGV
make[3]: *** [Makefile:437: gen/uc6502/tst_kyber-ntt.rel] Fehler 1
results/uc6502/tst_kyber-ntt.out:2:--- FAIL: cannot compile/link cases/tst_kyber-ntt.c
Failure: cases/tst_kyber-ntt.c
tst_kyber-ntt                       (f:  1, t:   1, c:    1, b:       0, T:         0)
Summary for 'uc6502': 1 failures, 36365 tests, 6373 test cases, 8938588 bytes, 392669958 ticks
   Failure: cases/tst_kyber-ntt.

gdb backtrace:

Program received signal SIGSEGV, Segmentation fault.
0x000000010048d038 in m6502_freeReg (reg=reg@entry=0x100711408 <m6502_regs>) at ralloc.c:114
114       if (m6502_reg_x->isFree)
(gdb) bt
#0  0x000000010048d038 in m6502_freeReg (reg=reg@entry=0x100711408 <m6502_regs>) at ralloc.c:114
#1  0x000000010048eb54 in m6502_assignRegisters (ebbi=0x1008deae0) at ralloc.c:1562
#2  0x000000010009ea4c in eBBlockFromiCode (ic=<optimized out>) at SDCCopt.c:4106
#3  0x00000001000bff70 in createFunction (name=0x1008c7090, body=0x1008cb9c0) at SDCCast.c:7854
#4  0x0000000100062300 in yyparse () at SDCC.y:2575
#5  0x000000010005bde8 in main (argc=20, argv=0x7ffffffff068, envp=<optimized out>) at SDCCmain.c:2940