Use the distributed ARM ar in libpico, not system (#2721)

Fixes #508

Author: earlephilhower

tools/libpico/CMakeLists.txt

@@ -312,8 +312,8 @@ target_link_libraries(ipv4-ipv6-bt-big-${cpu}
 
 foreach(tgt pico ipv4 ipv4-ipv6 ipv4-big ipv4-ipv6-big ipv4-bt ipv4-ipv6-bt ipv4-bt-big ipv4-ipv6-bt-big)
     add_custom_command(TARGET ${tgt}-${cpu} POST_BUILD
-        COMMAND ar d lib${tgt}-${cpu}.a stdio.c.obj stdio_uart.c.obj stdio_usb.c.obj stdio_usb_descriptors.c.obj pico_malloc.c.obj newlib_interface.c.obj
-        COMMAND ar d lib${tgt}-${cpu}.a btstack_flash_bank.c.obj  # Need to override with our own implementation
+        COMMAND arm-none-eabi-ar d lib${tgt}-${cpu}.a stdio.c.obj stdio_uart.c.obj stdio_usb.c.obj stdio_usb_descriptors.c.obj pico_malloc.c.obj newlib_interface.c.obj
+        COMMAND arm-none-eabi-ar d lib${tgt}-${cpu}.a btstack_flash_bank.c.obj  # Need to override with our own implementation
         COMMAND cp lib${tgt}-${cpu}.a ../../../lib/${cpu}/lib${tgt}.a
     )
 endforeach()