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.
33bc1ebe37Andrew Kelley committed on 10/5/2020, 3:10:47 AMparentd0a4110