shlib: specify the sysroot to find the correct crt1 (#441)

Author: yamt

Makefile

@@ -506,7 +506,8 @@ PIC_OBJS = \
 # to CC.  This is a workaround for a Windows command line size limitation.  See
 # the `%.a` rule below for details.
 $(SYSROOT_LIB)/%.so: $(OBJDIR)/%.so.a $(BUILTINS_LIB)
-	$(CC) -nodefaultlibs -shared -o $@ -Wl,--whole-archive $< -Wl,--no-whole-archive $(BUILTINS_LIB)
+	$(CC) -nodefaultlibs -shared --sysroot=$(SYSROOT) \
+	-o $@ -Wl,--whole-archive $< -Wl,--no-whole-archive $(BUILTINS_LIB)
 
 $(OBJDIR)/libc.so.a: $(LIBC_SO_OBJS) $(MUSL_PRINTSCAN_LONG_DOUBLE_SO_OBJS)