remove spurious MAP_32BIT definition from riscv64 arch

this was apparently copied from x86_64; it's not part of the kernel
API for riscv64. this change eliminates the need for a
riscv64-specific bits header and lets it use the generic one.

arch/riscv64/bits/mman.h

@@ -1 +0,0 @@
-#define MAP_32BIT       0x40