[AArch64][llvm-jitlink] Fix two QNX cross-compile build failures (NFC) (#206290)
Fixes two build failures when cross-compiling LLVM for QNX
(aarch64-unknown-nto-qnx8.0.0).
gcc `-Wstringop-overread` false-positive in `AArch64InstructionSelector.cpp`:
GCC's static analysis misreads the SmallVector fill constructor in
`selectUnmergeValues()` as potentially reading `NumInsertRegs *
sizeof(Register)`
bytes from the 16-byte inline buffer of `SmallVector<Register, 4>`,
emitting:
```
warning: reading between 20 and 17179869180 bytes from a region of size 16 [-Wstringop-overread]
```
Replacing the constructor-then-move with `assign()` fills the already-live object in place,
which GCC can analyze correctly.
### 2. `llvm-jitlink` — undefined references to socket functions on QNX
On QNX, POSIX socket functions (`getaddrinfo`, `socket`, `connect`, `freeaddrinfo`, `gai_strerror`)
live in `libsocket` rather than libc. Without it the QNX cross-compile linker fails.
Mirrors the existing `target_link_libraries` pattern already present for SunOS and Haiku.
Assisted-by: Claude Sonnet 4.6 <noreply@anthropic.com>