Fix concatenation of FE/BE compilation log strings

Due to the mixed usage of `std::string` objects and C-strings, null terminator
characters sometimes propagate into possible `program::updateBuildLog()`
inputs. In particular, `TranslationOutput::frontendCompilerLog` and
`::backendCompilerLog` get set this way within the `CompilerInterface::build()`
API implementation.

This becomes an issue whenever each of CL -> IR and IR -> ISA compilation steps
emits their warnings and/or errors:
`clGetProgramBuildInfo(..., CL_PROGRAM_BUILD_LOG, ...)` also relies on
`std::string` to C-string conversion, so the output is trimmed at the first
null terminator, i.e. the BE part of the build log simply gets lost.

The change handles possible null terminators within `program::updateBuildLog()`
and adds relevant regression tests.

Related-To: IGC-6509
Signed-off-by: Artem Gindinson <artem.gindinson@intel.com>

opencl/source/program/program.cpp

@@ -278,7 +278,8 @@ cl_int Program::getSource(std::string &binary) const {
 
 void Program::updateBuildLog(uint32_t rootDeviceIndex, const char *pErrorString,
                              size_t errorStringSize) {
-    ConstStringRef errorString(pErrorString, errorStringSize);
+    auto isTerminator = [](char c) { return c == '\0'; };
+    auto errorString = ConstStringRef(pErrorString, errorStringSize).trimEnd(isTerminator);
     if (errorString.empty()) {
         return;
     }