Try using gcc asm with tcc in ed25519-donna-64

The tiny c compiler (TCC) claims to support GCC-style asm statements. If that's so, then this patch may successfully get it to build ed25519-donna for X86_64. If not, then at least this patch should improve the error message that you get when your compiler doesn't have a uint128_t, by telling you what to do to "fix" the issue.

Closes #40240 (closed); bugfix on 0.2.9.4-alpha.