Leaner TIS-CI tests

Leaner TIS-CI tests

TIS-CI tests take forever.  It's annoying and cost them compute power
for little benefit.  A quick assesment of Monocypher reveals that the
only things we really care about are endianness and main word size.
Things like the sign of `char`, `sizeof(int)`, or `long double` are
mostly (or entirely) irrelevant.

So, all platforms supported by TIS-CI, only 5 are relevant:

- 16-bits little endian (I chose x86_16)
- 32-bits little endian (I chose x86_32)
- 32-bits big    endian (I chose sparc_32)
- 32-bits little endian (I chose rv64ifdq)
- 32-bits big    endian (I chose mips_64)

All the others are redundant.

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Also added the missing v_x25519 test.