[flang] Add --gcc-toolchain and --gcc-install-dir options to flang. (llvm#87360)

The `--gcc-toolchain` and `--gcc-install-dir` option were previously only visible to the Clang driver, but not Flang. These determine which assembler, linker, and libraries to use, e.g. for cross-compiling, and therefore are relevant for Flang as well.

Tests are implemented using a mock GCC installation in `basic_cross_linux_tree` copied over from Clang's tests. The Clang driver already contains tests with `--driver-mode=flang` but `flang-new` is an entirely different executable (containing the `-fc1` stage) that should be tested as well. While not all files in `basic_cross_linux_tree` are strictly needed for testing those two driver flags, they will be necessarily needed for future added flags such as `--rtlib`.
 
Also remove the entry `*.o` in flang's `.gitignore` since `crt*.o` files are needed in the GCC mock installation.

Fixes llvm#86729

Author: Meinersbur

clang/include/clang/Driver/Options.td

@@ -802,9 +802,11 @@ def B : JoinedOrSeparate<["-"], "B">, MetaVarName<"<prefix>">,
     HelpText<"Search $prefix$file for executables, libraries, and data files. "
     "If $prefix is a directory, search $prefix/$file">;
 def gcc_install_dir_EQ : Joined<["--"], "gcc-install-dir=">,
+  Visibility<[ClangOption, FlangOption]>,
   HelpText<"Use GCC installation in the specified directory. The directory ends with path components like 'lib{,32,64}/gcc{,-cross}/$triple/$version'. "
   "Note: executables (e.g. ld) used by the compiler are not overridden by the selected GCC installation">;
 def gcc_toolchain : Joined<["--"], "gcc-toolchain=">, Flags<[NoXarchOption]>,
+  Visibility<[ClangOption, FlangOption]>,
   HelpText<"Specify a directory where Clang can find 'include' and 'lib{,32,64}/gcc{,-cross}/$triple/$version'. "
   "Clang will use the GCC installation with the largest version">;
 def gcc_triple_EQ : Joined<["--"], "gcc-triple=">,

flang/.gitignore

@@ -5,7 +5,6 @@ build
 root
 tags
 TAGS
-*.o
 .nfs*
 *.sw?
 *~

flang/test/Driver/Inputs/basic_cross_linux_tree/usr/bin/i386-unknown-linux-gnu-as

@@ -0,0 +1 @@
+#!/bin/true

flang/test/Driver/Inputs/basic_cross_linux_tree/usr/bin/i386-unknown-linux-gnu-ld

@@ -0,0 +1 @@
+#!/bin/true

flang/test/Driver/Inputs/basic_cross_linux_tree/usr/bin/i386-unknown-linux-gnu-ld.bfd

@@ -0,0 +1 @@
+#!/bin/true

flang/test/Driver/Inputs/basic_cross_linux_tree/usr/bin/i386-unknown-linux-gnu-ld.gold

@@ -0,0 +1 @@
+#!/bin/true

flang/test/Driver/Inputs/basic_cross_linux_tree/usr/bin/x86_64-unknown-linux-gnu-as

@@ -0,0 +1 @@
+#!/bin/true

flang/test/Driver/Inputs/basic_cross_linux_tree/usr/bin/x86_64-unknown-linux-gnu-ld

@@ -0,0 +1 @@
+#!/bin/true

flang/test/Driver/Inputs/basic_cross_linux_tree/usr/bin/x86_64-unknown-linux-gnu-ld.bfd

@@ -0,0 +1 @@
+#!/bin/true

flang/test/Driver/Inputs/basic_cross_linux_tree/usr/bin/x86_64-unknown-linux-gnu-ld.gold

@@ -0,0 +1 @@
+#!/bin/true