add ENDBR64 instructions for BTI

Index: libass/x86/x86inc.asm
--- libass/x86/x86inc.asm.orig
+++ libass/x86/x86inc.asm
@@ -756,6 +756,9 @@ BRANCH_INSTR jz, je, jnz, jne, jl, jle, jnl, jnle, jg,
     %endif
     align function_align
     %2:
+    %if ARCH_X86_64
+        endbr64
+    %endif
     RESET_MM_PERMUTATION        ; needed for x86-64, also makes disassembly somewhat nicer
     %xdefine rstk rsp           ; copy of the original stack pointer, used when greater alignment than the known stack alignment is required
     %assign stack_offset 0      ; stack pointer offset relative to the return address
@@ -777,6 +780,9 @@ BRANCH_INSTR jz, je, jnz, jne, jl, jle, jnl, jnle, jg,
         global current_function %+ %1
     %endif
     %1:
+    %if ARCH_X86_64
+        endbr64
+    %endif
 %endmacro
 
 %macro cextern 1