[Driver] Always use gas with -fno-integrated-as on Solaris (#65489)

`clang -fno-integrated-as` doesn't currently work on Solaris: it doesn't
even select between 32 and 64-bit objects. Besides, Solaris has both the
native assembler (`/usr/bin/as`) and the GNU assembler (`/usr/bin/gas`
resp. `/usr/gnu/bin/as`). The native sparc and x86 assemblers aren't
compatible with `clang`'s assembler syntax to varying degrees, and the
command line options for `as` and `gas` are completely different.

Therefore this patch chooses to always use `gas` on Solaris, using
`gnutools::Assembler::ConstructJob` to pass the correct options.

Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86_64-pc-linux-gnu`.