Object files for C++ input files are placed right next to source files

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If one adds a C++ file to a JIT invocation, the temporary object file is placed right next to the source file (which likely resides in some version-controlled directory, not even a build dir). The name of the object file is not unique. If multiple JIT jobs use the C++ file this will lead to races on the output file where different jobs might remove each others temporary file.

// Initially we expect four files (stdout, stderr, log, input).
// @TEST-EXEC: test $(find . -type f | wc -l) -eq 4
//
// JIT the C++ file and keep the output file. It should end in the temp dir.
// @TEST-EXEC: spicyc -jT %INPUT
//
// We still expect four files (stdout, stderr, log, input).
// @TEST-EXEC: test $(find . -type f | wc -l) -eq 4

void f() {}

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Also backported to release/1.5 and release/1.6.