INTERP doesn't tail-recurse w/Clang 12

[snej]

The current macOS x86-64 machine code for INTERP is:

0000000100011950	pushq	%rbx
0000000100011951	movq	%rdi, %rbx
0000000100011954	movq	(%rdi), %rax
0000000100011957	leaq	0x8(%rax), %rdi
000000010001195b	callq	*(%rax)
000000010001195d	testq	%rax, %rax
0000000100011960	je	0x10001196e
0000000100011962	leaq	0x10(%rbx), %rdi
0000000100011966	movq	%rax, %rsi
0000000100011969	callq	*0x8(%rbx)
000000010001196c	popq	%rbx
000000010001196d	retq
000000010001196e	xorl	%eax, %eax
0000000100011970	popq	%rbx
0000000100011971	retq

The call to the next word (at 01969) isn't tail-call optimized. That means the stack will grow as multiple INTERPs are called.

This should be fixed when built with a Clang that supports the musttail attribute (Clang 13?)