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

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 #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

Start by reviewing PR #155072 and the current riscv64gc-unknown-fuchsia target configuration, then compare its triple prefix and baseline against the RVA20 and RVA22 ISA strings listed here. Determine the intended baseline and update the target metadata so the prefix and mandatory extensions are consistent; done means the target no longer advertises an incomplete or misleading baseline.