Move PluginArchitecture to common

Along with the accompanying function to parse a PE32 file. We're going
to have to define some common tags for different plugin types.

src/common/plugins.cpp

@@ -0,0 +1,53 @@
+#include "plugins.h"
+
+#include <fstream>
+#include <sstream>
+
+namespace fs = boost::filesystem;
+
+PluginArchitecture find_vst_architecture(fs::path plugin_path) {
+    std::ifstream file(plugin_path, std::ifstream::binary | std::ifstream::in);
+
+    // The linker will place the offset where the PE signature is placed at the
+    // end of the MS-DOS stub, at offset 0x3c
+    uint32_t pe_signature_offset;
+    file.seekg(0x3c);
+    file.read(reinterpret_cast<char*>(&pe_signature_offset),
+              sizeof(pe_signature_offset));
+
+    // The PE32 signature will be followed by a magic number that indicates the
+    // target architecture of the binary
+    uint32_t pe_signature;
+    uint16_t machine_type;
+    file.seekg(pe_signature_offset);
+    file.read(reinterpret_cast<char*>(&pe_signature), sizeof(pe_signature));
+    file.read(reinterpret_cast<char*>(&machine_type), sizeof(machine_type));
+
+    constexpr char expected_pe_signature[4] = {'P', 'E', '\0', '\0'};
+    if (pe_signature !=
+        *reinterpret_cast<const uint32_t*>(expected_pe_signature)) {
+        throw std::runtime_error("'" + plugin_path.string() +
+                                 "' is not a valid .dll file");
+    }
+
+    // These constants are specified in
+    // https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#machine-types
+    switch (machine_type) {
+        case 0x014c:  // IMAGE_FILE_MACHINE_I386
+            return PluginArchitecture::vst_32;
+            break;
+        case 0x8664:  // IMAGE_FILE_MACHINE_AMD64
+        case 0x0000:  // IMAGE_FILE_MACHINE_UNKNOWN
+            return PluginArchitecture::vst_64;
+            break;
+    }
+
+    // When compiled without optimizations, GCC 9.3 will warn that the function
+    // does not return if we put this in a `default:` case instead.
+    std::ostringstream error_msg;
+    error_msg << "'" << plugin_path
+              << "' is neither a x86 nor a x86_64 PE32 file. Actual "
+                 "architecture: 0x"
+              << std::hex << machine_type;
+    throw std::runtime_error(error_msg.str());
+}