build system: implement computeOutFilenames using std.zig.binNameAlloc
build system: implement computeOutFilenames using std.zig.binNameAlloc

See #6552 #6553 #6483

Also fixes a regression introduced in
1d777e99588be047ec4d7650f048b043d6e52da0 of not converting an integer to
a string for a command line parameter.

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