Don't use no-c++-exceptions version of libstdc++ (earlephilhower#280)

Per earlephilhower#276, earlephilhower#277, earlephilhower#274 there seems to be an issue with interrupts and some
other low-level ARM operations when using the distributed libstdc++.

Use the default libstdc++ built with the toolchain, instead.

Fixes earlephilhower#277
Fixes earlephilhower#276
Fixes earlephilhower#274

Update README.md

Add pico-debug support (earlephilhower#239)

Rename assembly dir to boot2, only has boot2.Ss (earlephilhower#230)

 Add CPU cycle counter accessors (earlephilhower#226)

Use the 24-bit SYSTICK peripheral, wrapped in logic to extend it to a
full 32 or 64bits.  W/o the wrapper, SYSTICK will wrap around in ~100ms.

Adds rp2040.getCycleCount() and rp2040.getCycleCount64()

Clean up the libpico build process as crt0.S from the pico-sdk should
be directly used.

Clean up the keywords file.

Remove accidental debug message

Clear I2C status on short slave reads

Fixes earlephilhower#222

The HW needs to have the TXC_ABRT flag cleared when a slave transmission is
cut short by the master, or else it will effectively break the I2C bus and
never recover.

Fix file creation on SD and SDFS (earlephilhower#216)

The O_CREAT/etc. flags on the Pico are a full 32-bits in size, but the
core was generating a flag for SdFat using an 8-bit type, so all the
O_CREAT, O_TRUNC, O_APPEND, etc. flags got cut off.

Fix the flag size.

Fixes earlephilhower#214

Fix SerialUART flush (earlephilhower#215)

Serial1.flush/Serial2.flush was not waiting for the proper FIFO to clear.
Use the proper call from the Pico SDK.

Thanks to Peter Remias for noting it.

Initial Arduino Nano RP2040 Connect support (earlephilhower#212)

Thanks to @Claupio for the bringup!
Reference earlephilhower#208