[PATCH v4] rust: Respect HOSTCC when linking for host

[Alice Ryhl]

Matthew Maurer <mmaurer@xxxxxxxxxx> writes:
> Currently, rustc defaults to invoking `cc`, even if `HOSTCC` is defined,
> resulting in build failures in hermetic environments where `cc` does not
> exist. This includes both hostprogs and proc-macros.
> 
> Since we are setting the linker to `HOSTCC`, we set the linker flavor to
> `gcc` explicitly. The linker-flavor selects both which linker to search
> for if the linker is unset, and which kind of linker flags to pass.
> Without this flag, `rustc` would attempt to determine which flags to
> pass based on the name of the binary passed as `HOSTCC`. `gcc` is the
> name of the linker-flavor used by `rustc` for all C compilers, including
> both `gcc` and `clang`.
> 
> Signed-off-by: Matthew Maurer <mmaurer@xxxxxxxxxx>

Tested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>