jcc: remove C-specific -std= flags when trying to compile as C++.

[oliverchang]

If a C -std value, e.g. -std=c99 is passed to clang++, the compiler will complain.

We should remove these flags when we are attempting to compile as C++ here:

oss-fuzz/infra/base-images/base-builder/jcc/jcc.go

Line 65 in bf577d8

cppBin := "clang++"

We should remove any variation of -std= that does not have the string "++" in it.