libbpf: support llvm-generated indirect jumps

For v5 instruction set, LLVM now is allowed to generate indirect
jumps for switch statements. Every such a jump will be accompanied
by necessary metadata (enabled by -emit-jump-table-sizes-section).
The -bpf-min-jump-table-entries llvm option may be used to control
the minimal size of a jump table which will be converted to an
indirect jumps.

For a given switch the following data will be generated by LLVM:
.rodata will contain actual addresses, .llvm_jump_table_sizes
and .rel.llvm_jump_table_sizes tables provide meta-data necessary
to find and relocate the offsets.

The code generated by LLVM for a switch will look, approximately,
like this:

    0: rX <- jump_table_x[i]
    2: rX *= rX
    3: gotox rX

This code will be rejected by the verifier as is. First
transformation required here is that jump_table_x at the insn(0)
actually points to the `.rodata` section (map). So for such loads
libbpf should create a proper map of type BPF_MAP_TYPE_INSN_ARRAY,
using the aforementioned meta-data.

Then, in the insn(2) the address in rX gets dereferenced to point to
an actual instruction address. (From the verifier's point of view,
the rX changes type from PTR_TO_MAP_VALUE to PTR_TO_INSN.)

The final line generates an indirect jump. The
format of the indirect jump instruction supported by BPF is

    BPF_JMP|BPF_X|BPF_JA, SRC=0, DST=Rx, off=0, imm=fd(M)

and, obviously, the map M must be the same map which was used to
init the register rX. This patch implements this in the following,
hacky, but so far suitable for all existing use-cases, way. On
encountering a `gotox` instruction libbpf tracks back to the
previous direct load from map and stores this map file descriptor
in the gotox instruction.

Signed-off-by: Anton Protopopov <[email protected]>

Author: aspsk