RISC-V: Use newlib nano specific libm.

The libm gamma functions use the _gamma_signgam field of the reentrant
structure, which changes offset with the --enable-newlib-reent-small
configure option, which means we need to use a newlib nano specific
version of libm in addition to libc in the nano.specs file.  Reported
by Keith Packard.  There is a riscv-gnu-toolchain patch that goes
along with this to create the new libm_nano.a file.

Signed-off-by: Jim Wilson <jimw@sifive.com>