Fix execl/execle/execlp varargs argument corruption

[gigachadmma69]

Summary

Fixes a bug where my_execl(), my_execle(), and my_execlp() always read the last vararg instead of each argument in sequence.

The argument-filling loop used getVargN(emu, cnt+1) (constant index — always the last arg before NULL) instead of getVargN(emu, i+1) (incrementing index — each arg in order). This corrupted the argv array passed to execv/execve/execvp.

Before (broken):

for(int i=0; i<cnt; ++i)
    argv[i] = getVargN(emu, cnt+1);  // always reads last arg

After (fixed):

for(int i=0; i<cnt; ++i)
    argv[i] = getVargN(emu, i+1);    // reads each arg in order

For example, execl("./game", "-fullscreen", "-nosound", NULL) would produce argv = [NULL, NULL] (reading past the args) instead of argv = ["-fullscreen", "-nosound"].

Fixes #3542

Test plan

• Compile on ARM64
• Test with an x64 binary that calls execl() to re-exec itself

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