[Driver] Add option to force undefined symbols during linking in BareMetal toolchain object. (#132807)

Add support for `-u` option to force defined symbols. This option is
supported by both lld and gnuld.

This is done as a part of the effort to merge RISCVToolchain object into
BareMetal toolchain object.

This is the 4th patch in the series of patches for merging
RISCVToolchain object into BareMetal toolchain object.

RFC:
https://discourse.llvm.org/t/merging-riscvtoolchain-and-baremetal-toolchains/75524

Author: quic-garvgupt

clang/lib/Driver/ToolChains/BareMetal.cpp

@@ -609,8 +609,9 @@ void baremetal::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     }
   }
 
-  Args.addAllArgs(CmdArgs, {options::OPT_L, options::OPT_T_Group,
-                            options::OPT_s, options::OPT_t, options::OPT_r});
+  Args.addAllArgs(CmdArgs,
+                  {options::OPT_L, options::OPT_u, options::OPT_T_Group,
+                   options::OPT_s, options::OPT_t, options::OPT_r});
 
   TC.AddFilePathLibArgs(Args, CmdArgs);
 

clang/test/Driver/baremetal-undefined-symbols.c

@@ -0,0 +1,14 @@
+// Check the arguments are correctly passed
+
+// Make sure -T is the last with gcc-toolchain option
+// RUN: %clang -### --target=riscv32 --gcc-toolchain= -Xlinker --defsym=FOO=10 -T a.lds -u foo %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-LD %s
+// CHECK-LD: {{.*}} "--defsym=FOO=10" {{.*}} "-u" "foo" {{.*}} "-T" "a.lds"
+
+// TODO: Merge this test with the above in the last patch when finally integrating riscv
+// Make sure -T is the last with gcc-toolchain option
+// RUN: %clang -### --target=aarch64-none-elf --gcc-toolchain= -Xlinker --defsym=FOO=10 -T a.lds -u foo %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-ARM-LD %s
+// RUN: %clang -### --target=armv6m-none-eabi --gcc-toolchain= -Xlinker --defsym=FOO=10 -T a.lds -u foo %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-ARM-LD %s
+// CHECK-ARM-LD: {{.*}} "-T" "a.lds" "-u" "foo" {{.*}} "--defsym=FOO=10"