springbok: make crt0.s portable

The `.rodata` psuedo-op is not a portable psuedo-op as gas does not
support it.  Explicitly replace it with the equivalent explicit section
switch, indicating that the section is an allocated section that has
program specific contents.  This repairs the build with gcc.

Change-Id: Ibc669d6cf9d178dd0da946175f61577036f25e3c