BUG: idxmin() fails for nullable integer data type (Int64)
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample

```python
import pandas as pd

s = pd.Series([1, 2, 3, 4], dtype="Int64")
s.idxmax()
```

#### Problem description

Currently, the `idxmax` call fails with
```python-traceback
  File "~lib/python3.8/site-packages/pandas/core/series.py", line 2110, in idxmax
    i = nanops.nanargmax(com.values_from_object(self), skipna=skipna)
  File "~lib/python3.8/site-packages/pandas/core/nanops.py", line 64, in _f
    raise TypeError(
TypeError: reduction operation 'argmax' not allowed for this dtype
```
being raised. This may be a duplicate of #32749, although the focus there seems to be generating the exception I already encountered. 

#### Expected Output
Should be `3`, I guess.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.5.13-arch2-1
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : de_DE.UTF-8
LOCALE           : de_DE.UTF-8

pandas           : 1.0.3
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 41.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : 2.4.4
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.2.0
numexpr          : None
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : 0.8.6
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : None
numba            : None
None

</details>
