Fix failures in AllocationTests/MappedMemoryTest.* on aarch64:

    Failing Tests (8):
        LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.AllocAndRelease/3
        LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.DuplicateNear/3
        LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.EnabledWrite/3
        LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.MultipleAllocAndRelease/3
        LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.SuccessiveNear/3
        LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.UnalignedNear/3
        LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.ZeroNear/3
        LLVM-Unit :: Support/./SupportTests/AllocationTests/MappedMemoryTest.ZeroSizeNear/3

Upstream-Issue: https://bugs.llvm.org/show_bug.cgi?id=14278#c10

--- a/lib/Support/Unix/Memory.inc
+++ b/lib/Support/Unix/Memory.inc
@@ -54,7 +54,7 @@
       llvm::sys::Memory::MF_EXEC:
     return PROT_READ | PROT_WRITE | PROT_EXEC;
   case llvm::sys::Memory::MF_EXEC:
-#if defined(__FreeBSD__)
+#if defined(__FreeBSD__) || (defined(__linux__) && defined(__aarch64__))
     // On PowerPC, having an executable page that has no read permission
     // can have unintended consequences.  The function InvalidateInstruction-
     // Cache uses instructions dcbf and icbi, both of which are treated by
@@ -62,6 +62,7 @@
     // executing these instructions will result in a segmentation fault.
     // Somehow, this problem is not present on Linux, but it does happen
     // on FreeBSD.
+    // The same problem is present even on aarch64 Linux (at least on musl).
     return PROT_READ | PROT_EXEC;
 #else
     return PROT_EXEC;