Document all supported options for --arch and --os

We support a specific set of architectures and operating system
configurations, and we should let the user know which ones these are.
Else the user has to dig into the source code to reverse-engineer it
from config.cpp.

Fixes: #5267
Fixes: #5268

Author: tautschnig

src/util/config.h

@@ -82,15 +82,25 @@ class symbol_tablet;
   "(gcc)"                                                                      \
 
 #define HELP_CONFIG_PLATFORM \
-  " --arch                       set architecture (default: "                  \
+  " --arch <arch>                set architecture (default: "                  \
                                  << configt::this_architecture() << ")\n"      \
-  " --os                         set operating system (default: "              \
+  "                              to one of: alpha, arm, arm64, armel, armhf,\n"\
+  "                              hppa, i386, ia64, mips, mips64, mips64el,\n"  \
+  "                              mipsel, mipsn32, mipsn32el, powerpc, ppc64,\n"\
+  "                              ppc64le, riscv64, s390, s390x, sh4, sparc,\n" \
+  "                              sparc64, v850, x32, x86_64, or none\n"        \
+  " --os <os>                    set operating system (default: "              \
                                  << configt::this_operating_system() << ")\n"  \
+  "                              to one of: freebsd, linux, macos, solaris,\n" \
+  "                              or windows\n"                                 \
   " --i386-linux, --i386-win32, --i386-macos, --ppc-macos\n"                   \
   "   --win32, --winx64          set architecture and operating system\n"      \
-  " --16, --32, --64             set width of int\n"                           \
   " --LP64, --ILP64, --LLP64,\n"                                               \
-  "   --ILP32, --LP32            set width of int, long and pointers\n"        \
+  "   --ILP32, --LP32            set width of int, long and pointers, but\n"   \
+  "                              don't override default architecture and\n"    \
+  "                              operating system\n"                           \
+  " --16, --32, --64             equivalent to --LP32, --ILP32, --LP64 (on\n"  \
+  "                              Windows: --LLP64)\n"                          \
   " --little-endian              allow little-endian word-byte conversions\n"  \
   " --big-endian                 allow big-endian word-byte conversions\n"     \
   " --gcc                        use GCC as preprocessor\n"                    \