BaseTools: Align include guards policy

BaseTools: Align include guards policy

The EDK II C Coding Standards Specification states that:
"Names starting with one or two underscores, such as
_MACRO_GUARD_FILE_NAME_H_, must not be used. They are
reserved for compiler implementation." [1]

The Ecc tool currently checks that the include guard end with
a trailing underscore. Thus, the check and the error message
should both be modified.

The new check forces having one sole trailing underscore
character, as the example in the specification shows:
"FILE_NAME_H_" [1]
This would allow to have more consistency.

[1] Section 5.3.5 "All include file contents must be protected
by a #include guard":
https://edk2-docs.gitbook.io/
edk-ii-c-coding-standards-specification/5_source_files/53_include_files

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Sami Mujawar <Sami.Mujawar@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>