[RISCV] Don't allow X0 to be used for 'r' constraint in inline assembly

Some instructions treat x0 as a special encoding rather than as a
value of 0. Since we don't parse the inline assembly to know what
the instruction is, chooser the safest option of never using x0.

Fixes #63747.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D154744