bpf, arm64: Mark dummy_tramp as global

When building with clang + CONFIG_CFI_CLANG=y, the following error
occurs at link time:

  ld.lld: error: undefined symbol: dummy_tramp

dummy_tramp is declared globally in C but its definition in inline
assembly does not use .global, which prevents clang from properly
resolving the references to it when creating the CFI jump tables.

Mark dummy_tramp as global so that the reference can be properly
resolved.

Fixes: b2ad54e ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
Suggested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1661
Link: https://lore.kernel.org/bpf/20220713173503.3889486-1-nathan@kernel.org

arch/arm64/net/bpf_jit_comp.c

@@ -604,6 +604,7 @@ void dummy_tramp(void);
 
 asm (
 "	.pushsection .text, \"ax\", @progbits\n"
+"	.global dummy_tramp\n"
 "	.type dummy_tramp, %function\n"
 "dummy_tramp:"
 #if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL)