linux/i386: Make syscall6 more robust and correct
linux/i386: Make syscall6 more robust and correct

LLVM10 exposed a subtle flaw in the previous implementation that made
the mmap tests fail.

raulgrell/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,zig-lang/zig,zig-lang/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig