pmc_save_user_callchain() emits the pc it just loaded before checking
whether fp is the ABI's zero frame-chain terminator. At the bottom of
a well-formed chain under _start, fp comes back 0 as expected, but the
paired pc is stale rtld data left on the stack -- a legal userspace VA
that still passes PMC_IN_USERSPACE(), so it gets emitted as a bogus
extra frame. This shows up in flame graphs as a spurious hex-valued
root frame below _start.

Check fp == 0 alongside the existing checks before emitting, matching
how arm/arm64/powerpc already load the next fp before their check.

Measured via 1kHz hwpmc sampling on an OCA: stacks with any unresolved
hex frame drop from 23.9% to 1.3%, and stacks with hex at the root drop
from 5.5% to 0.3%.

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