[Issue 24815] New: betterC can not use alloca()

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24815

          Issue ID: 24815
           Summary: betterC can not use alloca()
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dfawcus+dlang at employees.org

The following program fails to link due to alloca being a missing symbol,
despite it being being declared in core.stdc.stdlib.  The program compiles and
runs with both GDC and LDC.

This is with: DMD64 D Compiler v2.109.1

```D
import core.stdc.stdio;
import core.stdc.string;
import core.stdc.stdlib;

extern(C) void main() {
        int len = 4;
        char *mem = cast(char*)alloca(len);
        memcpy(mem, cast(const char *)"Foo\0", len);
        char[] memS = mem[0..len];

        printf("%s; %.*s\n", memS.ptr, cast(int)memS.length, memS.ptr);
}
```

```
$ dmd -betterC alloca1.d
/usr/bin/ld: alloca1.o: in function `main':
alloca1.d:(.text.main[main]+0x21): undefined reference to `__alloca'
collect2: error: ld returned 1 exit status
Error: undefined reference to `__alloca`
       referenced from `main`
       perhaps a library needs to be added with the `-L` flag or `pragma(lib,
...)`
Error: linker exited with status 1
       cc alloca1.o -o alloca1 -m64 -Xlinker --export-dynamic
-L/usr/lib/x86_64-linux-gnu -lpthread -lm -lrt -ldl 
```

A similar error is seen when using -m32.

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