Change the DateTimeFormatterBuilder::FIELD_MAP type from Map to Array

[wenshao]

The DateTimeFormatterBuilder::FIELD_MAP is of type Map<Character, TemporalField>, with key values in the range 'A-Z' and 'a-z'.
We can change the type of FIELD_MAP to an array of length 128 and then use char as an index to access the array.
This will reduce the memory usage of DateTimeFormatterBuilder::FIELD_MAP and improve performance slightly.
By analyzing the jmap dump memory, we can see that the memory size used by DateTimeFormatterBuilder::FIELD_MAP has dropped from 1.12kb to 528b, the number of objects has decreased and the memory has become more flat. as shown below:
before:
after:
In order to make it easier for reviewers to see my ideas, I submitted a draft Pull Request ( https://github.com/openjdk/jdk/pull/26634 <https://github.com/openjdk/jdk/pull/26634 > ). Please give me feedback.
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