Pass CLANGSA_CXXFLAGS to clangsa tests (#54337)
`CLANGSA_CXXFLAGS` are defined in `Make.inc`,
https://github.com/JuliaLang/julia/blob/f712512a1159771d9a61e1a768e1a8b297a493c8/Make.inc#L1501
and it is included in the Makefile for clangsa tests.
https://github.com/JuliaLang/julia/blob/f712512a1159771d9a61e1a768e1a8b297a493c8/test/clangsa/Makefile#L4
But it does not get passed to clangsa tests properly. This PR fixes
this.
