MdeModulePkg: 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
MdeModulePkg.

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>

MdeModulePkg/Library/UefiBootManagerLib/BmBoot.c

@@ -1448,7 +1448,7 @@ BmExpandLoadFile (
       "%a:%a: failed to allocate reserved pages: "
       "BufferSize=%Lu LoadFile=\"%s\" FilePath=\"%s\"\n",
       gEfiCallerBaseName,
-      __FUNCTION__,
+      __func__,
       (UINT64)BufferSize,
       LoadFileText,
       FileText