Use max(1, Sys.CPU_THREADS) BLAS threads for apple aarch64. (#46085)

(cherry picked from commit 97df6db)

Author: chriselrod

stdlib/LinearAlgebra/src/LinearAlgebra.jl

@@ -583,7 +583,11 @@ function __init__()
     Base.at_disable_library_threading(() -> BLAS.set_num_threads(1))
 
     if !haskey(ENV, "OPENBLAS_NUM_THREADS")
-        BLAS.set_num_threads(max(1, Sys.CPU_THREADS ÷ 2))
+        @static if Sys.isapple() && Base.BinaryPlatforms.arch(Base.BinaryPlatforms.HostPlatform()) == "aarch64"
+            BLAS.set_num_threads(max(1, Sys.CPU_THREADS))
+        else
+            BLAS.set_num_threads(max(1, Sys.CPU_THREADS ÷ 2))
+        end
     end
 end