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
