Symlink libgcc_s on Linux when USE_SYSTEM_CSL=1

This adds `libgcc_s` as one of the libraries that gets symlinked when
`USE_SYSTEM_CSL=1`, which is necessary since we load it from our
`DEP_LIBS` RPATH-substitution list.

base/Makefile

@@ -215,8 +215,8 @@ else ifneq ($(USE_SYSTEM_OPENLIBM),0)
 $(eval $(call symlink_system_library,OPENLIBM,$(LIBMNAME)))
 endif
 
-# On macOS, libgcc_s has soversion 1.1 always on aarch64 and only for GCC 12+
-# (-> libgfortran 5) on x86_64
+# On macOS, libgcc_s has soversion 1.1 always on aarch64,
+# but only for GCC 12+ (e.g. libgfortran 5) on x86_64
 ifeq ($(OS),Darwin)
 ifeq ($(ARCH),aarch64)
 $(eval $(call symlink_system_library,CSL,libgcc_s,1.1))
@@ -227,7 +227,12 @@ else
 $(eval $(call symlink_system_library,CSL,libgcc_s,1))
 endif
 endif
+
+# Linux always has soversion 1
+else ifeq($(OS),Linux)
+$(eval $(call symlink_system_library,CSL,libgcc_s,1))
 endif
+
 ifneq (,$(LIBGFORTRAN_VERSION))
 $(eval $(call symlink_system_library,CSL,libgfortran,$(LIBGFORTRAN_VERSION)))
 endif