Fix linux link error when using LLVM reactor backend.

Fix linux link error when using LLVM reactor backend.

Seen building on Ubuntu 18.04.1:

```
libllvm.a(DynamicLibrary.cpp.o): In function `llvm::sys::DynamicLibrary::HandleSet::~HandleSet()':
DynamicLibrary.cpp:(.text._ZN4llvm3sys14DynamicLibrary9HandleSetD2Ev+0x1c): undefined reference to `dlclose'
DynamicLibrary.cpp:(.text._ZN4llvm3sys14DynamicLibrary9HandleSetD2Ev+0x2c): undefined reference to `dlclose'
libllvm.a(DynamicLibrary.cpp.o): In function `llvm::sys::DynamicLibrary::HandleSet::DLSym(void*, char const*)':
DynamicLibrary.cpp:(.text._ZN4llvm3sys14DynamicLibrary9HandleSet5DLSymEPvPKc+0x1): undefined reference to `dlsym'
```

Also removes --no-as-needed hack.

Bug: b/123360006
Change-Id: Icef1078e68cbf68af991e52f7fbb6e8d449e0809
Reviewed-on: https://swiftshader-review.googlesource.com/c/24310
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>