[test-release.sh] Set TEST_SUITE_HOST_CC to the release testing build compiler when...

[test-release.sh] Set TEST_SUITE_HOST_CC to the release testing build compiler when compiling test-suite tools.

The tools used by test-suite are originally configured to compile with cc by
default, and this is dictated by TEST_SUITE_HOST_CC.
However, it is possible that on some systems that the version of cc may either
not be present or it may not be able to compile the tools as it may be too old,
which could be an issue seen during release testing.

This patch updates the compiler to be the default build compiler that is used
for release testing. If no such compiler it specified, then cc will be set as
the test-suite tools build compiler by default (as it already is set under
TEST_SUITE_HOST_CC).

Differential Revision: https://reviews.llvm.org/D118357