Fixed native build toolchain and compiler prefix

* When build=native toolchain_prefix and compiler_prefix must be
  set to "" instead of auto

* Resolves COMPMID-5395

Change-Id: I481ee4596e16d55d2a696af9124a0d06c916cef7
Signed-off-by: Pablo Marquez Tello <pablo.tello@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/7855
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Gunes Bayir <gunes.bayir@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Benchmark: Arm Jenkins <bsgcomp@arm.com>
diff --git a/SConstruct b/SConstruct
index be438e0..53cdc3b 100644
--- a/SConstruct
+++ b/SConstruct
@@ -347,17 +347,14 @@
         elif env['os'] == 'tizen':
             auto_toolchain_prefix = "aarch64-tizen-linux-gnu-"
 
-if env['build'] == 'native':
-    toolchain_prefix = ""
-
-if env["toolchain_prefix"] == "":
+if env['build'] == 'native' or env["toolchain_prefix"] == "":
     toolchain_prefix = ""
 elif env["toolchain_prefix"] == "auto":
     toolchain_prefix = auto_toolchain_prefix
 else:
     toolchain_prefix = env["toolchain_prefix"]
 
-if env["compiler_prefix"] == "":
+if env['build'] == 'native' or env["compiler_prefix"] == "":
     compiler_prefix = ""
 elif env["compiler_prefix"] == "auto":
     compiler_prefix = toolchain_prefix