arch/avr: Remove -nostartfiles -nodefaultlibs from LDFLAGS

These options are not ld options.
The recent versions of ld complain on them.

https://sourceware.org/pipermail/binutils/2021-June/116826.html

arch/avr/src/avr/Toolchain.defs

@@ -94,26 +94,22 @@ endif
 
 ifeq ($(CONFIG_AVR_TOOLCHAIN),BUILDROOT)
   CROSSDEV ?= avr-nuttx-elf-
-  LDFLAGS += -nostartfiles -nodefaultlibs
 endif
 
 # AVR CrossPack under macOS
 
 ifeq ($(CONFIG_AVR_TOOLCHAIN),CROSSPACK)
   CROSSDEV ?= avr-
-  LDFLAGS += -nostartfiles -nodefaultlibs
 endif
 
 # GCC toolchain under Linux
 
 ifeq ($(CONFIG_AVR_TOOLCHAIN),LINUXGCC)
   CROSSDEV ?= avr-
-  LDFLAGS += -nostartfiles -nodefaultlibs
 endif
 
 ifeq ($(CONFIG_AVR_TOOLCHAIN),WINAVR)
   CROSSDEV ?= avr-
-  LDFLAGS += -nostartfiles -nodefaultlibs
 endif
 
 # Default toolchain