include/flatbuffers/base.h: fix no_sanitize issue with old clang (#5610)

include/flatbuffers/base.h: fix no_sanitize issue with old clang (#5610)

Older clang versions raise:
```
./flatbuffers/base.h:365:1: error: unknown attribute 'no_sanitize' ignored [-Werror,-Wattributes]
__supress_ubsan__("alignment")
^
./flatbuffers/base.h:246:50: note: expanded from macro '__supress_ubsan__'
  #define __supress_ubsan__(type) __attribute__((no_sanitize(type)))
                                                 ^
```

Comparing https://releases.llvm.org/3.6.0/tools/clang/docs/AttributeReference.html
with https://releases.llvm.org/3.7.0/tools/clang/docs/AttributeReference.html
shows that __attribute__((no_sanitize(type))) is available since 3.7.0