rust: Fix RUSTFLAGS being added as prefixed linker flags

[tytan652]

Fix an issue where RUSTFLAGS is added as linker flags to cc.

"1 basic" ran with RUSTFLAGS=-Cforce-frame-pointers=yes:

[4/4] Compiling Rust source '../test cases/rust/1 basic/subdir/prog.rs'
FAILED: [code=1] subdir/rust-program2 
rustc -C linker=cc --color=always -C debug-assertions=no -C overflow-checks=no -C embed-bitcode=no --crate-type bin -Cdefault-linker-libraries -C link-arg=-Wl,--allow-shlib-undefined -C link-arg=-Cforce-frame-pointers=yes -g -Cforce-frame-pointers=yes --crate-name rust_program2 --emit dep-info=subdir/rust-program2.p/rust-program2.d --emit link=subdir/rust-program2 -C metadata=bdf6c15@@rust-program2@exe '../test cases/rust/1 basic/subdir/prog.rs'
error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "-m64" "/home/*removed*/meson/b 06c574b6a1/subdir/rustc1UtRwy/symbols.o" "<2 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,libcfg_if-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,liblibc-*,librustc_std_workspace_core-*,liballoc-*,libcore-*,libcompiler_builtins-*}.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-L" "/home/*removed*/meson/b 06c574b6a1/subdir/rustc1UtRwy/raw-dylibs" "-fuse-ld=lld" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-o" "subdir/rust-program2" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-Wl,--allow-shlib-undefined" "-Cforce-frame-pointers=yes"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: cc: error: unrecognized command-line option '-Cforce-frame-pointers=yes'; did you mean '-fomit-frame-pointer'?

Many of the rust test will fail with RUSTFLAGS being set with flags that the cc linker will not understand.

Also add a test which is a copy of basic with RUSTFLAGS set.

[tytan652]

I ponder if I should go deeper and find whereRUSTFLAGS ends up in get_external_link_args/'rust_link_args'…