cmd/link: use only the configured C compiler in TestCGOLTO
The test had been assuming that any 'gcc' or 'clang' command found in
$PATH could be used to compile cgo dependencies for the target GOARCH
and GOOS. That assumption does not hold in general: for example,
the GOARCH/GOOS configuration may be cross-compiling, which will cause
the test to fail if the native 'gcc' and/or 'clang' is not configured
for the target architecture.
Instead, leave the 'CC' variable unset and assume only that the user
has configured it appropriate to the environment in which they are
running the test.