[swift-inspect] support CMake build for Linux

[andrurogerz]

Purpose

Enable building swift-inspect on Linux using CMake.

Overview

• Update CMakeLists.txt to include Linux libraries and sources
• Add -Xcc -D_GNU_SOURCE to CMAKE_Swift_FLAGS so process_vm_readv is visible
• Add #pragma once to system headers to avoid recursive include
• Update README.md with Linux CMake instructions

Background

PR #77938 added Linux support to swift-inspect, but only supported building with SwiftPM. This PR brings it to parity with Windows which also supports CMake build.

Validation

Built on Fedora 41 workstation with CMake Version 3.30.5 and Swift version 6.02.

$ cd ~/src/swift
$ cmake -S ./tools/swift-inspect -B ./build -G Ninja -D ArgumentParser_DIR=~/src/swift-argument-parser/build/cmake/modules -D CMAKE_Swift_FLAGS="-Xcc -I$(git rev-parse --show-toplevel)/include/swift/SwiftRemoteMirror"
$ cmake --build build

[compnerd]

@swift-ci please test