fix compiler avxvnniint16 detection

[nihui]

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 95.90%. Comparing base (d2c1bd1) to head (8cf0831). :warning: Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6108      +/-   ##
==========================================
+ Coverage   95.60%   95.90%   +0.29%     
==========================================
  Files         840      840              
  Lines      265625   265625              
==========================================
+ Hits       253955   254735     +780     
+ Misses      11670    10890     -780     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions[bot]]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15155464	15155464	0 :kissing_heart:
armhf	6182208	6182208	0 :kissing_heart:
aarch64	9520608	9520608	0 :kissing_heart:

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}