riscv64gc-unknown-fuchsia: rva22+v baseline is inconsistent with triple prefix and incomplete

[jchzhou]

When reading the release notes of Rust 1.96, I noticed that the riscv64gc-unknown-fuchsia target recently changed its baseline from rv64gc to rva22+v in PR #155072. This raises two concerns:

1. The riscv64gc- triple prefix implies a baseline of mandatory extensions in the RISC-V RVA20 profile. Promoting the baseline to RVA22+Vector while keeping the triple prefix is probably misleading.
2. The only change in PR Set the Fuchsia ABI to the documented minimum #155072 is adding +v,. This change does not include any mandatory extension by the RVA22 profile, but only an optional extension: vector.

As a reference, here are the longer ISA strings of RVA20 and RVA22:

RVA20U64: rv64imafdc_ziccamoa_ziccif_zicclsm_ziccrse_zicntr_zicsr_zmmul_za128rs_zaamo_zalrsc_zca_zcd
RVA20S64: rv64imafdc_ziccamoa_ziccif_zicclsm_ziccrse_zicntr_zicsr_zifencei_zmmul_za128rs_zaamo_zalrsc_zca_zcd_ssccptr_sstvala_sstvecd_svade_svbare
RVA22U64: rv64imafdcb_zic64b_zicbom_zicbop_zicboz_ziccamoa_ziccif_zicclsm_ziccrse_zicntr_zicsr_zihintpause_zihpm_zmmul_za64rs_zaamo_zalrsc_zfhmin_zca_zcd_zba_zbb_zbs_zkt
RVA22S64: rv64imafdcb_zic64b_zicbom_zicbop_zicboz_ziccamoa_ziccif_zicclsm_ziccrse_zicntr_zicsr_zifencei_zihintpause_zihpm_zmmul_za64rs_zaamo_zalrsc_zfhmin_zca_zcd_zba_zbb_zbs_zkt_ssccptr_sscounterenw_sstvala_sstvecd_svade_svbare_svinval_svpbmt

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cc @dramforever