tools uapi: fix RISC-V 64-bit support

The BPF library is not built on 64-bit RISC-V, as the BPF feature is
not detected. Looking more in details, feature/test-bpf.c fails to build
with the following error:

| In file included from /tmp/linux-4.19.12/tools/include/uapi/asm/bitsperlong.h:17,
|                  from /tmp/linux-4.19.12/tools/include/uapi/asm-generic/unistd.h:2,
|                  from /usr/include/riscv64-linux-gnu/asm/unistd.h:1,
|                  from test-bpf.c:2:
| /tmp/linux-4.19.12/tools/include/asm-generic/bitsperlong.h:14:2: error: #error Inconsistent word size. Check asm/bitsperlong.h
|  #error Inconsistent word size. Check asm/bitsperlong.h
|   ^~~~~

The UAPI from the tools directory is missing RISC-V support, therefore
bitsperlong.h from asm-generic is used, defaulting to 32 bits.

Fix that by adding tools/arch/riscv/include/uapi/asm/bitsperlong.h as
a copy of arch/riscv/include/uapi/asm/bitsperlong.h and by updating
tools/include/uapi/asm/bitsperlong.h.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>