[Driver] Stop searching other prefixes once a GCC installation is found in one prefix

so that when --sysroot is specified, the detected GCC installation will not be
overridden by another from /usr which happens to have a larger version.

This behavior is particularly inconvenient when the system has a larger version
GCC while the user wants to try out an older sysroot.

Delete some tests from linux-ld.c which overlap with cross-linux.c