[BPF] introduce __attribute__((bpf_fastcall)) (#105417)

This commit introduces attribute bpf_fastcall to declare BPF functions
that do not clobber some of the caller saved registers (R0-R5).

The idea is to generate the code complying with generic BPF ABI,
but allow compatible Linux Kernel to remove unnecessary spills and
fills of non-scratched registers (given some compiler assistance).

For such functions do register allocation as-if caller saved registers
are not clobbered, but later wrap the calls with spill and fill
patterns that are simple to recognize in kernel.

For example for the following C code:

    #define __bpf_fastcall __attribute__((bpf_fastcall))

    void bar(void) __bpf_fastcall;
    void buz(long i, long j, long k);

    void foo(long i, long j, long k) {
      bar();
      buz(i, j, k);
    }

First allocate registers as if:

    foo:
      call bar    # note: no spills for i,j,k (r1,r2,r3)
      call buz
      exit

And later insert spills fills on the peephole phase:

    foo:
      *(u64 *)(r10 - 8) = r1;  # Such call pattern is
      *(u64 *)(r10 - 16) = r2; # correct when used with
      *(u64 *)(r10 - 24) = r3; # old kernels.
      call bar
      r3 = *(u64 *)(r10 - 24); # But also allows new
      r2 = *(u64 *)(r10 - 16); # kernels to recognize the
      r1 = *(u64 *)(r10 - 8);  # pattern and remove spills/fills.
      call buz
      exit

The offsets for generated spills/fills are picked as minimal stack
offsets for the function. Allocated stack slots are not used for any
other purposes, in order to simplify in-kernel analysis.

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