cmd/link: return correct default linker for the platform
If no external linker was passed with -extld, link currently assumes
that it is "gcc" which is not correct for platforms that use clang
toolchain. Return "clang" for platforms that use it, this fixes dir
tests on freebsd/riscv64.
For #53466
Change-Id: Ie3bce1b9581839d0b3b2129908355cd30ae9a713
Reviewed-on: https://go-review.googlesource.com/c/go/+/432756
Run-TryBot: Cherry Mui <cherryyz@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Joel Sing <joel@sing.id.au> Reviewed-by: Mikaël Urankar <mikael.urankar@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Joedian Reid <joedian@golang.org>