use "gcc" instead of "cc" on *-sun-solaris systems when linking

use "gcc" instead of "cc" on *-sun-solaris systems when linking

On illumos and Solaris systems, Rust will use GCC as the link editor.
Rust does this by invoking "cc", which on many (Linux and perhaps BSD)
systems is generally either GCC or a GCC-compatible front-end.  On
historical Solaris systems, "cc" was often the Sun Studio compiler.
This history casts a long shadow, and as such, even most modern
illumos-based operating systems tend to install GCC as "gcc", without
also making it available as "cc".

We should invoke GCC as "gcc" on such systems to ensure we get the right
compiler driver.