Allow tfcompile_flags to be a list (#14337)

This PR is a follow up to 12769 and 14333 to address the issue
raised in 12767 so that it is possible to specify the flag with
```
tfcompile_flags = ["--target_cpu='core-avx2'", "--xla_enable_fast_math=false"]`
```
Previously it was only possible to specify the flag with:
```
tfcompile_flags = "--target_cpu='core-avx2' --xla_enable_fast_math=false"
```

This PR will fix 12767.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>