pico-sdk doesn't work when compiling with picolibc

[keith-packard]

There are several compatibility issues when trying to build with picolibc version 1.8.11:

• Using GCC adds --specs=nosys.specs, a newlib-specific file
• pico-sdk assumes the C library will define __printflike, picolibc defines __picolibc_format instead.
• GCC's -Wredundant-decls complains about the symbol aliasing used to define stdout and stderr.
• Picolibc will add another linker script unless the application uses the -T format instead of -Wl,--script.
• Picolibc uses different constructor symbols (__bothinit_array_start and __bothinit_array_end).
• Picolibc needs thread-local variables to be per-core instead of global.
• When using picolibc's printf, we want to inherit the project printf feature selection