configure: Support cross compiling with clang

When a user specifies the compiler with --cc assume it's already
fully named, even if the user also specifies a cross-prefix. This
allows clang to be selected for the compiler, which doesn't use
prefixes, but also still provide a cross prefix for binutils. If
a user needs a prefix on the compiler that they specify with --cc,
then they'll just have to specify it with the prefix prepended.

Also ensure user provided cflags are used when testing the compiler,
since the flags may drastically change behavior, such as the --target
flag for clang.

With these changes it's possible to cross compile for riscv with
clang after configuring with

 ./configure --arch=riscv64 --cc=clang --cflags='--target=riscv64' \
             --cross-prefix=riscv64-linux-gnu-

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>