Use findstring instead of direct equality checks for LLVM_CPUTYPE
conditions to better handle ARCHCPUEXTFLAGS that may contain additional
ISA extensions. This makes the CPU type detection more robust when
dealing with extended instruction sets while still ensuring the required
base ISA extensions are present.
For example, ARCHCPUEXTFLAGS="imc_zicsr_zifencei" will now correctly
match as sifive-e20 rather than failing the exact match check.
Signed-off-by: Huang Qi <huangqi3@xiaomi.com>