BaseTools: Build against C++14 when building with clang

clang 17 defaults to C++17, where the 'register' keyword is deprecated
and the warning changed to an error. To avoid build errors, compile
against C++14 by specifying '-std=c++14' in CXXFLAGS.

Signed-off-by: Rebecca Cran <rebecca@quicinc.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>