When load() in x86_64-gen.c generates a zero constant for a 64-bit
register, it uses:
o(0xc031 + REG_VALUE(r) * 0x900);
REG_VALUE(r) masks to (r & 7), losing bit 3, and no orex() call
emits the REX prefix needed for registers r8-r15. For TREG_R11
(used by gcall_or_jmp for indirect calls), this emits
"xor %ebx,%ebx" (31 db) instead of "xor %r11d,%r11d" (45 31 db),
clobbering the wrong register.
The test compiles an indirect call through a null function pointer
((void(*)(void))0)() via libtcc, then inspects the generated machine
code for the incorrect encoding.
