avr: drop GNU toolchain dependency

- Use compiler-rt and picolibc instead of avr-libc.
  - Use ld.lld instead of avr-ld (or avr-gcc).

This makes it much easier to get started with TinyGo on AVR because
installing these extra tools (gcc-avr, avr-libc) can be a hassle.
It also opens the door for future improvements such as ThinLTO.

There is a code size increase but I think it's worth it in the long run.
The code size increase can hopefully be reduced with improvements to the
LLVM AVR backend and to compiler-rt.

targets/avr.ld

@@ -5,6 +5,8 @@ MEMORY
     RAM (xrw)       : ORIGIN = 0x800000 + __ram_start, LENGTH = __ram_size
 }
 
+ENTRY(__vector_RESET)
+
 SECTIONS
 {
     .text :
@@ -12,9 +14,11 @@ SECTIONS
         KEEP(*(.vectors))
         KEEP(*(.text.__vector_RESET))
         KEEP(*(.text.main)) /* main must follow the reset handler */
+        *(.text)
         *(.text.*)
         *(.progmem)
         *(.progmem.*)
+        . = ALIGN(16); /* needed with ld.lld for some reasoon */
     }
 
     .stack (NOLOAD) :