macOS <=10.14: move build flags to OSLIBS (JuliaLang#50017)

Replace file src/mach_dyld_atfork.tbd with command line flags

Fixes JuliaLang#49976

Author: benz0li

src/Makefile

@@ -150,7 +150,7 @@ endif
 CLANG_LDFLAGS := $(LLVM_LDFLAGS)
 ifeq ($(OS), Darwin)
 CLANG_LDFLAGS += -Wl,-undefined,dynamic_lookup
-OSLIBS += $(SRCDIR)/mach_dyld_atfork.tbd
+OSLIBS += -Wl,-U,__dyld_atfork_parent -Wl,-U,__dyld_atfork_prepare -Wl,-U,__dyld_dlopen_atfork_parent -Wl,-U,__dyld_dlopen_atfork_prepare
 LIBJULIA_PATH_REL := @rpath/libjulia
 else
 LIBJULIA_PATH_REL := libjulia