lint: warn about pointer casts between different kinds of types

Pointer casts from an integer type to a floating-point type and vice
versa get a 'maybe troublesome' warning now. The previous assump

lint: warn about pointer casts between different kinds of types

Pointer casts from an integer type to a floating-point type and vice
versa get a 'maybe troublesome' warning now. The previous assumption
that all types of the same bit-size are convertible may have been valid
from a technical point of view, but still such code should get more
attention.

The rules for struct and union types could be made more fine-grained
later, if the need arises. To suppress this warning, it's always
possible to cast to an intermediate 'void *'.

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tests/lint: fix a few ilp32 tests

tests/lint: spell platform identifiers for 'long double' consistently

The test file names don't have a hyphen, so the identifiers shouldn't
have one either.

tests/lint: add platform-specific tests for troublesome pointer casts