ArmPkg: Update code to be more C11 compliant by using __func__

__FUNCTION__ is a pre-standard extension that gcc and Visual C++ among
others support, while __func__ was standardized in C99.

Since it's more standard, replace __FUNCTION__ with __func__ throughout
ArmPkg.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

ArmPkg/Library/LinuxBootBootManagerLib/LinuxBootBm.c

@@ -151,7 +151,7 @@ PlatformBootManagerAfterConsole (
       LOAD_OPTION_ACTIVE
       );
   } else {
-    DEBUG ((DEBUG_ERROR, "%a: PcdLinuxBootFileGuid was not set!\n", __FUNCTION__));
+    DEBUG ((DEBUG_ERROR, "%a: PcdLinuxBootFileGuid was not set!\n", __func__));
   }
 }