Kernel/aarch64: Add helper to get the current TTBR0_EL1

Author: https://github.com/FireFox317
Commit: https://github.com/SerenityOS/serenity/commit/9e90932bfb
Pull-request: https://github.com/SerenityOS/serenity/pull/16911
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/nico ✅

Kernel/Arch/aarch64/ASM_wrapper.h

@@ -24,6 +24,14 @@ inline void set_ttbr0_el1(FlatPtr ttbr0_el1)
     asm("msr ttbr0_el1, %[value]" ::[value] "r"(ttbr0_el1));
 }
 
+inline FlatPtr get_ttbr0_el1()
+{
+    FlatPtr ttbr0_el1;
+    asm("mrs %[value], ttbr0_el1\n"
+        : [value] "=r"(ttbr0_el1));
+    return ttbr0_el1;
+}
+
 inline void set_sp_el1(FlatPtr sp_el1)
 {
     asm("msr sp_el1, %[value]" ::[value] "r"(sp_el1));