[OpenMP][AIX] Not to create symbolic links to libomp.so in install step

[kkwli]

bb563b1 handles the links in the build directory but misses the case in the install step. This patch is to link only the libomp.a on AIX.

[madanial0]

LGTM. Thanks!

[llvm-ci]

LLVM Buildbot has detected a new failure on builder ppc64le-flang-rhel-clang running on ppc64le-flang-rhel-test while building openmp at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/42383

Here is the relevant piece of the build log for the reference

Step 6 (test-build-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
...
PASS: Flang :: Transforms/OpenACC/acc-implicit-data-fortran.F90 (3997 of 4007)
PASS: Flang :: Transforms/constant-argument-globalisation.fir (3998 of 4007)
PASS: Flang :: Transforms/external-name-interop-symref-array.fir (3999 of 4007)
PASS: Flang :: Transforms/debug-line-table.fir (4000 of 4007)
PASS: Flang :: Transforms/tbaa3.fir (4001 of 4007)
PASS: Flang :: Transforms/dlti-dependency.fir (4002 of 4007)
PASS: Flang :: Driver/omp-driver-offload.f90 (4003 of 4007)
PASS: Flang :: Transforms/debug-dwarf-version.fir (4004 of 4007)
PASS: Flang :: Driver/linker-options.f90 (4005 of 4007)
PASS: Flang :: Intrinsics/math-codegen.fir (4006 of 4007)
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=3044.909700