New update of the RISC-V big endian support. Changes since v3:
* Changed riscv_subword to take endianness into account. This fixed multiple issues with long long on 32-bit. Testsuite results are now on par with little endian also on 32 bit. The only exception is gcc.c-torture/compile/pr35318.c, which has an internal compiler error (test fail) on 32 bit big endian, but not on 32 bit little endian (test pass, even though the generated code looks suspicious). // Marcus