[PATCH 1/2] Support picolibc targets

Match *-picolibc-* and select picolibc as the default C library, plus continuing to use the newlib-based logic for other configuration items. Add custom spec file fragments for use with picolibc: * '--oslib='. Allows targets to insert an OS library after the C library in the LIB_PATH spec fil

[PATCH 1/2] Support picolibc targets

Match *-picolibc-* and select picolibc as the default C library, plus continuing to use the newlib-based logic for other configuration items. Add custom spec file fragments for use with picolibc: * '--oslib='. Allows targets to insert an OS library after the C library in the LIB_PATH spec fil