asm: allow negative constants for builtin function calls

asm: allow negative constants for builtin function calls

The work to encode a platform into various constant types made it
so that decoding a call to a builtin helper with a negative value
fails with

    decoding instructions for section <sectionname>:
    offset <offset>: invalid constant 0xffffffff

for a BPF instruction of "call -1". This is because we can't
represent -1 as a tagged platform constant.

Allow negative constants by not transforming them into a platform
constant at all. Adjust the platform tag size so that we never
generate a platform constant with the high bit set. This avoids
confusing it with a negative number when reinterpreting it as a
signed number and ensures that trying to marshal such an
instruction gives an error.

Fixes: https://github.com/cilium/ebpf/issues/1797
Signed-off-by: Lorenz Bauer <lmb@isovalent.com>