libcxx: Fix libcxx tests on aarch64 with libunwind

Some tests uses 'long double' to/from conversions and for some targets
they are provided by compiler runtime (either compiler-rt or libgcc).
However when building libcxx with linunwinder current test configuration
at target_info.py do not include the required libraries, as:

  not llvm_unwinder:
    "-lc++" "-lm" "-lgcc_s" "-lgcc" "-lpthread" "-lc" "-lgcc_s" "-lgcc"

  llvm_unwinder
    "-lc++" "-lm" "-lpthread" "-lc" "-lunwind" "-ldl"

This causes some tests build issues with missing symbols on aarch64,
for instance, where 'long double' is a binary float with 128-bits with
mostly of internal operations being provided by software routines.

This patch changes how to define the default linker flags with libunwinder by
adding libgcc regardless.

I checked and aarch64 and x86_64 with libcxx and libunwind (with and without
LIBCXXABI_USE_LLVM_UNWINDER).


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@279552 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/test/target_info.py

@@ -180,7 +180,8 @@ class LinuxLocalTI(DefaultTargetInfo):
         if llvm_unwinder:
             flags += ['-lunwind', '-ldl']
         else:
-            flags += ['-lgcc_s', '-lgcc']
+            flags += ['-lgcc_s']
+        flags += ['-lgcc']
         use_libatomic = self.full_config.get_lit_bool('use_libatomic', False)
         if use_libatomic:
             flags += ['-latomic']