cmd/compile: libFuzzer instrumentation fakePC overflow on 386 arch [1.19 backport]

[gopherbot]

@randall77 requested issue #56141 to be considered for backport to the next 1.19 minor release.

@gopherbot please open a backport to 1.19. This bug causes libfuzzer mode to fail to compile. There is no easy workaround.
(I don't think this applies to 1.18.)

[gopherbot]

Change https://go.dev/cl/442435 mentions this issue: [release-branch.go1.19] all: prevent fakePC overflow on 386 in libfuzzer mode

[gopherbot]

Closed by merging e73130c to release-branch.go1.19.