altos: Move from newlib-nano to picolibc

Use picolibc.ld, which involved some .ld file hacking, including:

 1) Defining ao_boot RAM address in the .ld file and declaring it 'extern'
 2) Changing how m0 interrupt vector got moved to ram
 3) Using -Taltos.ld instead of -Wl,-Taltos.ld so picolibc.specs wouldn't add picolibc.ld
 4) Placing romconfig vars in '.init.1' and '.init.2' sections instead of '.romconfig'
 5) Place code needing to run out of RAM in section .srodata instead of .ramtext

Signed-off-by: Keith Packard <keithp@keithp.com>