libbpf: Add __NR_bpf definition for LoongArch

LoongArch uses the generic syscall table, where __NR_bpf is defined
as 280 in include/uapi/asm-generic/unistd.h.

To align with other architectures, add the __NR_bpf definition for
LoongArch to avoid a potential compilation failure: "error __NR_bpf
not defined. libbpf does not support your arch."

This is a follow up patch of:

  commit b0c47807d31d ("bpf: Add sparc support to tools and samples.")
  commit bad1926dd2f6 ("bpf, s390: fix build for libbpf and selftest suite")
  commit ca31ca8247e2 ("tools/bpf: fix perf build error with uClibc (seen on ARC)")
  commit e32cb12ff52a ("bpf, mips: Fix build errors about __NR_bpf undeclared")

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260526063936.16769-1-yangtiezhu@loongson.cn

src/bpf.c

@@ -59,6 +59,8 @@
 #  define __NR_bpf 6319
 # elif defined(__mips__) && defined(_ABI64)
 #  define __NR_bpf 5315
+# elif defined(__loongarch__)
+#  define __NR_bpf 280
 # else
 #  error __NR_bpf not defined. libbpf does not support your arch.
 # endif