[cxx] Remove use of register keyword. (#18057)
[cxx] Remove use of register keyword.
https://jenkins.mono-project.com/job/test-mono-pull-request-wasm/19408/parsed_console/log_content.html
```
/mnt/jenkins/workspace/test-mono-pull-request-wasm/mono/utils/mono-md5.c:259:2: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
register guint32 a, b, c, d;
^~~~~~~~~
/mnt/jenkins/workspace/test-mono-pull-request-wasm/mono/utils/mono-md5.c:259:2: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
register guint32 a, b, c, d;
^~~~~~~~~
/mnt/jenkins/workspace/test-mono-pull-request-wasm/mono/utils/mono-md5.c:259:2: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
register guint32 a, b, c, d;
^~~~~~~~~
/mnt/jenkins/workspace/test-mono-pull-request-wasm/mono/utils/mono-md5.c:259:2: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
register guint32 a, b, c, d;
^~~~~~~~~
4 warnings generated.
```