ghc-toolchain: Don't leave stranded a.outs when testing for -g0

This happened because, when ghc-toolchain tests for -g0, it does so by
compiling an empty program.  This compilation creates an a.out.

Since we create a temporary directory, lets place the test program
compilation in it also, so that it gets cleaned up.

Fixes: 25b0b404
Closes: ghc/ghc#25203