selftests/bpf: Set __BITS_PER_LONG if target is bpf for LoongArch

selftests/bpf: Set __BITS_PER_LONG if target is bpf for LoongArch

If target is bpf, there is no __loongarch__ definition, __BITS_PER_LONG
defaults to 32, __NR_nanosleep is not defined:

  #if defined(__ARCH_WANT_TIME32_SYSCALLS) || __BITS_PER_LONG != 32
  #define __NR_nanosleep 101
  __SC_3264(__NR_nanosleep, sys_nanosleep_time32, sys_nanosleep)
  #endif

Work around this problem, by explicitly setting __BITS_PER_LONG to
__loongarch_grlen which is defined by compiler as 64 for LA64.

This is similar with commit 36e70b9b06bf ("selftests, bpf: Fix broken
riscv build").

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1677585781-21628-1-git-send-email-yangtiezhu@loongson.cn