[CMake] Remove -Wl,-allow-shlib-undefined which was added in rL221530

In GNU ld, gold and LLD, --no-allow-shlib-undefined is the default when
linking an executable. The option disallows unresolved symbols in shared objects.
(gold and LLD catch fewer cases than GNU ld. See D57385 for details)
See D57569 why it is bad idea to use --allow-shlib-undefined for executables [a].

GNU ld traditionally copied DT_NEEDED entries transitively. This was
deemed not good, so GNU ld 2.22 defaulted to --no-copy-dt-needed-entries.
gold and LLD always behave like --no-copy-dt-needed-entries.
rL221530 added -Wl,-allow-shlib-undefined to make some old releases of GNU ld's
--no-copy-dt-needed-entries to actually work.

Due to [a] and [b], this patch drops -Wl,-allow-shlib-undefined.

[b]: In a -DBUILD_SHARED_LIBS=on build, `--as-needed --allow-shlib-undefined`
can unexpectedly suppress some .dynsym entries.  The issue can cause
mlir-cpu-runner to fail at runtime. Note, on Debian, gcc newer than (gcc-9-20190125-2) enable
--as-needed by default.
See https://sourceware.org/bugzilla/show_bug.cgi?id=26551 for a reduced example.

Reviewed By: mehdi_amini, echristo

Differential Revision: https://reviews.llvm.org/D86839

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