Auto merge of #79608 - alessandrod:bpf, r=nagisa

BPF target support

This adds `bpfel-unknown-none` and `bpfeb-unknown-none`, two new no_std targets that generate little and big endian BPF. The approach taken is very similar to the cuda target, where `TargetOptions::obj_is_bitcode` is enabled and code generation is done by the linker.

I added the targets to `dist-various-2`. There are [some tests](https://github.com/alessandrod/bpf-linker/tree/main/tests/assembly) in bpf-linker and I'm planning to add more. Those are currently not ran as part of rust CI.

compiler/rustc_span/src/symbol.rs

@@ -328,6 +328,7 @@ symbols! {
         box_free,
         box_patterns,
         box_syntax,
+        bpf_target_feature,
         braced_empty_structs,
         branch,
         breakpoint,
@@ -1332,6 +1333,7 @@ symbols! {
         wrapping_add,
         wrapping_mul,
         wrapping_sub,
+        wreg,
         write_bytes,
         xmm_reg,
         ymm_reg,