[flang] Add -rtlib flag (#99058)

This patch allows the -rtlib flag with flang-new to select between the
libgcc_s and compiler-rt runtimes. The behaviour is identical to the
same flag with clang.

Author: DavidTruby

clang/include/clang/Driver/Options.td

@@ -5622,7 +5622,7 @@ def resource_dir_EQ : Joined<["-"], "resource-dir=">, Flags<[NoXarchOption]>,
   Alias<resource_dir>;
 def rpath : Separate<["-"], "rpath">, Flags<[LinkerInput]>, Group<Link_Group>,
   Visibility<[ClangOption, CLOption, DXCOption, FlangOption]>;
-def rtlib_EQ : Joined<["-", "--"], "rtlib=">, Visibility<[ClangOption, CLOption]>,
+def rtlib_EQ : Joined<["-", "--"], "rtlib=">, Visibility<[ClangOption, CLOption, FlangOption]>,
   HelpText<"Compiler runtime library to use">;
 def frtlib_add_rpath: Flag<["-"], "frtlib-add-rpath">, Flags<[NoArgumentUnused]>,
   Visibility<[ClangOption, FlangOption]>,

flang/test/Driver/linker-flags.f90

@@ -11,6 +11,7 @@
 ! RUN: %flang -### --target=x86_64-unknown-dragonfly %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,UNIX,UNIX-F128%f128-lib
 ! RUN: %flang -### --target=x86_64-unknown-haiku %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,HAIKU,HAIKU-F128%f128-lib
 ! RUN: %flang -### --target=x86_64-windows-gnu %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,MINGW,MINGW-F128%f128-lib
+! RUN: %flang -### -rtlib=compiler-rt --target=aarch64-linux-gnu %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,UNIX,COMPILER-RT
 
 ! NOTE: Clang's driver library, clangDriver, usually adds 'oldnames' on Windows,
 !       but it is not needed when compiling Fortran code and they might bring in
@@ -33,6 +34,7 @@
 ! UNIX-F128LIBQUADMATH-SAME: "-lFortranFloat128Math" "--as-needed" "-lquadmath" "--no-as-needed"
 ! SOLARIS-F128LIBQUADMATH-SAME: "-lFortranFloat128Math" "-z" "ignore" "-lquadmath" "-z" "record"
 ! UNIX-SAME: "-lFortranRuntime" "-lFortranDecimal" "-lm"
+! COMPILER-RT: "{{.*}}{{\\|/}}libclang_rt.builtins.a"
 
 ! DARWIN-LABEL:  "{{.*}}ld{{(\.exe)?}}"
 ! DARWIN-SAME: "[[object_file]]"
@@ -61,3 +63,6 @@
 ! MSVC-LABEL: link
 ! MSVC-SAME: /subsystem:console
 ! MSVC-SAME: "[[object_file]]"
+
+! COMPILER-RT-NOT: "-lgcc"
+! COMPILER-RT-NOT: "-lgcc_s"