vlc_common: fix asm() usage with Clang in struct C11 mode
asm() is optional in C11/C17. CLang does support it in gnu11/gnu17 mode but not in c11/c17 mode. It should support it in these mode too...
Maybe the define should be set in the configure file so it's used when testing all the asm calls.
The cpuid call in the core assumes the asm C11 option is there to detect the CPU.