goto-cc: newlines in command files are just whitespace

GCC's doc says that options are separated by whitespace, with no
particular significance of newlines. Therefore, process the entire file
at once rather than line-by-line.

Fixes: #6592

Author: tautschnig

regression/goto-gcc/at_files/args

@@ -1 +1,3 @@
+-o
+test.gb
 other.c

src/goto-cc/gcc_cmdline.cpp

@@ -12,8 +12,9 @@ Author: CM Wintersteiger, 2006
 #include "gcc_cmdline.h"
 
 #include <cstring>
-#include <iostream>
 #include <fstream>
+#include <iostream>
+#include <sstream>
 
 #include <util/prefix.h>
 
@@ -256,16 +257,18 @@ bool gcc_cmdlinet::parse_arguments(
     if(has_prefix(argv_i, "@"))
     {
       std::ifstream opts_file(argv_i.substr(1));
+      std::ostringstream all_lines;
       std::string line;
 
       while(std::getline(opts_file, line))
-      {
-        // erase leading whitespace
-        line.erase(0, line.find_first_not_of("\t "));
+        all_lines << ' ' << line;
 
-        if(!line.empty())
-          parse_specs_line(line, false);
-      }
+      line = all_lines.str();
+      // erase leading whitespace
+      line.erase(0, line.find_first_not_of("\t "));
+
+      if(!line.empty())
+        parse_specs_line(line, false);
 
       continue;
     }