[Driver] Add option to force udnefined symbols during linking in BareMetal

toolchain object.

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

Change-Id: Ia6597c756923a77fd9c7cb9a6ae8e52a56f5457d

Author: quic-garvgupt

Diff for: clang/lib/Driver/ToolChains/BareMetal.cpp

@@ -566,8 +566,9 @@ 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);
 

Diff for: clang/test/Driver/riscv-args.c

@@ -0,0 +1,15 @@
+// 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"
+