The same commit from gnu-efi that changed CopyMem in a way that broke refind also changed SetMem.
This patch attempts to fix it in the same way upstream fixed CopyMem.

The commit: https://sourceforge.net/p/gnu-efi/code/ci/deb8a7f267d96299b9aa41300539f617db54c2a9/
The bug: https://bugs.gentoo.org/934474

--- a/gptsync/gptsync.c
+++ b/gptsync/gptsync.c
@@ -164,7 +164,7 @@ static UINTN write_mbr(VOID)
     }
     if (!have_bootcode) {
         // no boot code found in the MBR, add the syslinux MBR code
-        SetMem(sector, MBR_BOOTCODE_SIZE, 0);
+        refit_call3_wrapper(gBS->SetMem, sector, MBR_BOOTCODE_SIZE, 0);
         refit_call3_wrapper(gBS->CopyMem, sector, syslinux_mbr, SYSLINUX_MBR_SIZE);
     }
 
--- a/libeg/lodepng_xtra.c
+++ b/libeg/lodepng_xtra.c
@@ -87,7 +87,7 @@ int MyStrlen(const char *InString) {
 } // int MyStrlen()
 
 VOID *MyMemSet(VOID *s, int c, size_t n) {
-    SetMem(s, c, n);
+    MySetMem(s, c, n);
     return s;
 }
 
--- a/refind/config.c
+++ b/refind/config.c
@@ -667,7 +667,7 @@ VOID ReadConfig(CHAR16 *FileName)
             HandleStrings(TokenList, TokenCount, &(GlobalConfig.DriverDirs));
 
         } else if (MyStriCmp(TokenList[0], L"showtools")) {
-            SetMem(GlobalConfig.ShowTools, NUM_TOOLS * sizeof(UINTN), 0);
+            MySetMem(GlobalConfig.ShowTools, NUM_TOOLS * sizeof(UINTN), 0);
             GlobalConfig.HiddenTags = FALSE;
             for (i = 1; (i < TokenCount) && (i < NUM_TOOLS); i++) {
                 FlagName = TokenList[i];
--- a/refind/launch_legacy.c
+++ b/refind/launch_legacy.c
@@ -114,7 +114,7 @@ static EFI_STATUS ActivateMbrPartition(IN EFI_BLOCK_IO *BlockIO, IN UINTN Partit
     }
     if (!HaveBootCode) {
         // no boot code found in the MBR, add the syslinux MBR code
-        SetMem(SectorBuffer, MBR_BOOTCODE_SIZE, 0);
+        MySetMem(SectorBuffer, MBR_BOOTCODE_SIZE, 0);
         MyCopyMem(SectorBuffer, syslinux_mbr, SYSLINUX_MBR_SIZE);
     }
 
--- a/refind/lib.c
+++ b/refind/lib.c
@@ -651,7 +651,7 @@ static VOID SetFilesystemData(IN UINT8 *Buffer, IN UINTN BufferSize, IN OUT REFI
 
     LOG(2, LOG_LINE_NORMAL, L"Identifying filesystem types....");
     if ((Buffer != NULL) && (Volume != NULL)) {
-        SetMem(&(Volume->VolUuid), sizeof(EFI_GUID), 0);
+        MySetMem(&(Volume->VolUuid), sizeof(EFI_GUID), 0);
         Volume->FSType = FS_TYPE_UNKNOWN;
 
         if (BufferSize >= (1024 + 100)) {
--- a/refind/lib.h
+++ b/refind/lib.h
@@ -133,10 +133,13 @@ VOID MyFreePool(IN OUT VOID *Pointer);
 
 // When using GNU-EFI, call the EFI's built-in gBS->CopyMem() function, because
 // GNU-EFI 3.0.18 changed its CopyMem() definition in a way that broke rEFInd.
+// Same for SetMem
 #ifdef __MAKEWITH_GNUEFI
 #define MyCopyMem(Dest, Src, len) refit_call3_wrapper(gBS->CopyMem, Dest, Src, len)
+#define MySetMem(Dest, Src, len) refit_call3_wrapper(gBS->SetMem, Dest, Src, len)
 #else
 #define MyCopyMem(Dest, Src, len) CopyMem(Dest, Src, len)
+#define MySetMem(Dest, Src, len) SetMem(Dest, Src, len)
 #endif
 
 BOOLEAN EjectMedia(VOID);