pkg/csource: Force promotion of 64-bit constant values
pkg/csource: Force promotion of 64-bit constant values

Constant 64-bit arguments to the variadic syscall(2) must have their
width specified explicitly.  In practice this is not necessary most of
the time, but on amd64/freebsd with clang the compiler can and does
store the constant 32-bit value to the stack, leaving garbage in the
upper 32 bits.

This makes C reproducers somewhat uglier, but I see no other solution.

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