Make meson recognize the Qualcomm LLVM toolchain

Meson calls `path/to/clang++ --version` to guess which build toolchain the
user has picked to build the source code. For the Qualcomm LLVM toolchain,
the output have an unusual output as shown below:

```
clang version 8.0.12
Snapdragon LLVM ARM Compiler 8.0.12 (based on llvm.org 7.0+)
Target: arm-unknown-linux-gnueabi
Thread model: posix
Repository: (ssh://git-hexagon-aus.qualcomm.com:...)
InstalledDir: /pkg/qct/software/llvm/release/arm/8.0.12/bin
```

Another unusual pattern is the output of `path/to/ld.qcld --version`:

```
ARM Linker from Snapdragon LLVM ARM Compiler Version 8.0.12
ARM Linker based on LLVM version: 8.0
```

The Meson logic is modified accordingly so that Meson can correctly
determine toolchain as "LLVM aarch64 cross-compiler on GNU/Linux64 OS".

This is the expected output of
`meson --native-file native_file.ini --cross-file cross_file.ini build/aarch64-debug/`:

```
...
C++ compiler for the host machine: ... (clang 8.0.12 "clang version 8.0.12")
C++ linker for the host machine: ... ld.lld 8.0.12
...
```

1 files changed, 6 insertions, 0 deletions

diff --git a/mesonbuild/linkers.py b/mesonbuild/linkers.py
index fa30b9a..3ce7111 100644
--- a/mesonbuild/linkers.py
+++ b/mesonbuild/linkers.py
@@ -927,6 +927,12 @@ class ArmClangDynamicLinker(ArmDynamicLinker):
     def import_library_args(self, implibname: str) -> T.List[str]:
         return ['--symdefs=' + implibname]
 
+class QualcommLLVMDynamicLinker(LLVMDynamicLinker):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        # ARM Linker from Snapdragon LLVM ARM Compiler
+        self.id = 'ld.qcld'
 
 class PGIDynamicLinker(PosixDynamicLinkerMixin, DynamicLinker):