Remove references to `__riscv_zvlsseg`

[mr-c]

Sorry for the archaelogy here. Zvlsseg is an old extension name from pre-1.0 RVV. It was removed from RVV 1.0 and the last several compiler releases never define __riscv_zvlsseg.

Originally posted by @topperc in #1285 (comment)

[mr-c]

Thank you @topperc for your report; I guess llvm/llvm-project@7a275dc is the relevant commit for LLVM

Given @Ruhung 's PR that added these references in February, I'm guessing there are some vendor forks of clang out there where this check is still relevant. Any advice from either of you on how to move forward?

[mr-c]

@topperc Isn't our existing check flexible enough?

simde/simde/simde-arch.h

Lines 553 to 555 in 9130a9f

	#if defined(__riscv_zvlsseg) || defined(__riscv_v)
	# define SIMDE_ARCH_RISCV_ZVLSSEG 1
	#endif

[Ruhung]

Remove references to __riscv_zvlsseg.
#1353

[topperc]

@topperc Isn't our existing check flexible enough?

simde/simde/simde-arch.h

Lines 553 to 555 in 9130a9f

#if defined(__riscv_zvlsseg) || defined(__riscv_v)

define SIMDE_ARCH_RISCV_ZVLSSEG 1

#endif

Maybe I took the title of #1285 too literally. It says "only when we have __riscv_zvlsseg flag", but that's not what the code did.

[mr-c]

Shall we leave things as-is to support the older/forked clangs?