[Driver] Disable -faddrsig on Gentoo by default

Gentoo supports combining clang toolchain with GNU binutils, and many
users actually do that.  As -faddrsig is not supported by GNU strip,
this results in a lot of warnings.  Disable it by default and let users
enable it explicitly if they want it; with the intent of reevaluating
when the underlying feature becomes standarized.

See also: https://bugs.gentoo.org/667854

Differential Revision: https://reviews.llvm.org/D56047

llvm-svn: 350028