LoongArch: Fix internal error running "gcc -march=native" on LA664
On LA664, the PRID preset is ISA_BASE_LA64V110 but the base architecture
is guessed ISA_BASE_LA64V100. This causes a warning to be outputed:
cc1: warning: base architecture 'la64' differs from PRID preset '?'
But we've not set the "?" above in loongarch_isa_base_strings, thus it's
a nullptr and then an ICE is triggered.
Add ISA_BASE_LA64V110 to genopts and initialize
loongarch_isa_base_strings[ISA_BASE_LA64V110] correctly to fix the ICE.
The warning itself will be fixed later.