asm volatile with no instructions generates invalid C code (missing empty string template)

[RbPyer]

Describe the bug

When using asm volatile amd64 with only a memory clobber and no instructions, V generates invalid code that fails to compile with tcc/gcc.

Using nop as a placeholder instruction compiles successfully with tcc but generates unnecessary CPU instructions.

Reproduction Steps

module main

fn main() {
    asm volatile amd64 {
        ; ; ; memory
    }
}

Expected Behavior

This pattern is the standard way to express a compiler-only memory barrier on x86-64, where acquire/release fences don't require CPU instructions (due to TSO memory model) but still need to prevent compiler reordering.

Current Behavior

tcc:

Compilation with unknown C compiler
================== C compilation error (from tcc): ==============
cc: /tmp/v_1000/main.01KKCPGT9T6G9Y67QQ1ZJXGC8W.tmp.c:4581: error: string constant expected
=================================================================

gcc:

================== C compilation error (from gcc): ==============
cc: /tmp/v_1000/main.01KKCPTTB4YY157V228X3BMW5Q.tmp.c: В функции «newatomics__fence»:
cc: /tmp/v_1000/main.01KKCPTTB4YY157V228X3BMW5Q.tmp.c:4891:25: ошибка: expected string literal before «:» token
cc:  4891 |                         : : : "memory"
cc:       |                         ^
cc: /tmp/v_1000/main.01KKCPTTB4YY157V228X3BMW5Q.tmp.c:4896:25: ошибка: expected string literal before «:» token
cc:  4896 |                         : : : "memory"
cc:       |                         ^
cc: /tmp/v_1000/main.01KKCPTTB4YY157V228X3BMW5Q.tmp.c:4901:25: ошибка: expected string literal before «:» token
cc:  4901 |                         : : : "memory"
cc:       |                         ^
=================================================================

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.5.1 07ee934

Environment details (OS name and version, etc.)

V full version	V 0.5.1 ffc6eaf.07ee934
OS	linux, "EndeavourOS Linux"
Processor	32 cpus, 64bit, little endian, AMD Ryzen 9 9950X3D 16-Core Processor
Memory	34.25GB/60.37GB
V executable	/home/oswyndel/v/v
V last modified time	2026-03-10 19:45:07
V home dir	OK, value: /home/oswyndel/v
VMODULES	OK, value: /home/oswyndel/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/oswyndel
Git version	git version 2.53.0
V git status	07ee934
.git/config present	true
cc version	cc (GCC) 15.2.1 20260209
gcc version	gcc (GCC) 15.2.1 20260209
clang version	clang version 21.1.8
tcc version	tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 696c1d84
emcc version	N/A
glibc version	ldd (GNU libc) 2.43

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