more iszero for generic linear algebra

[stevengj]

Changed several x == 0 calls to iszero(x) in LinearAlgebra's generic.jl.

(I noticed this when someone asked about norm for a custom number type on discourse, and I found it wouldn't work until you defined promotion with Int.)

[oscardssmith]

Most of these would be solved by #43256, but that is blocked on me finishing it.

[stevengj]

Unrelated CI failure:

Error in testset LinearAlgebra/qr:
Error During Test at none:1
  Got exception outside of a @test
  ProcessExitedException(3)

and

FileWatching                       (10) \|    35.30 \|   3.28 \|  9.3 \|     132.33 \|  1115.29
--
  | Test                               (10) \|        started at 2022-01-28T14:35:44.424
  | From worker 15:	┌ Warning: Skipping hostname verification tests. Is `openssl` on the path?
  | From worker 15:	│   exception = IOError: could not spawn `openssl version`: no such file or directory (ENOENT)
  | From worker 15:	└ @ Main.Test2Main_LibGit2_libgit2.LibGit2Tests /cache/build/default-amdci4-6/julialang/julia-master/julia-55cd945124/share/julia/stdlib/v1.8/LibGit2/test/libgit2.jl:3104
  | From worker 15:	┌ Warning: Skipping hostname verification tests. Unable to determine a hostname which maps to the loopback address
  | From worker 15:	└ @ Main.Test2Main_LibGit2_libgit2.LibGit2Tests /cache/build/default-amdci4-6/julialang/julia-master/julia-55cd945124/share/julia/stdlib/v1.8/LibGit2/test/libgit2.jl:3134
  | LibGit2/libgit2                    (15) \|         failed at 2022-01-28T14:35:45.989
  | Error During Test at /cache/build/default-amdci4-6/julialang/julia-master/julia-55cd945124/share/julia/stdlib/v1.8/LibGit2/test/libgit2.jl:2224
  | Got exception outside of a @test
  | IOError: read: i/o error (EIO)