-march=rv64gc_xtheadvector does not define __riscv_vector

[nihui]

riscv-gnu-toolchain with the latest gcc

gcc defines __riscv_vector but clang does not
Can we keep it consistent and define __riscv_vector as well?
This will make it easier to port existing rvv code directly

$ ./riscv64-unknown-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=./riscv64-unknown-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/data/action/osd/riscv/libexec/gcc/riscv64-unknown-linux-gnu/15.0.0/lto-wrapper
Target: riscv64-unknown-linux-gnu
Configured with: /data/action/osd/riscv-gnu-toolchain/gcc/configure --target=riscv64-unknown-linux-gnu --prefix=/data/action/osd/riscv --with-sysroot=/data/action/osd/riscv/sysroot --with-pkgversion=g155da081706 --with-system-zlib --enable-shared --enable-tls --enable-languages=c,c++,fortran --disable-libmudflap --disable-libssp --disable-libquadmath --disable-libsanitizer --disable-nls --disable-bootstrap --src=.././gcc --disable-default-pie --disable-multilib --with-abi=lp64d --with-arch=rv64imafdc --with-tune=rocket --with-isa-spec=20191213 'CFLAGS_FOR_TARGET=-O2    -mcmodel=medlow' 'CXXFLAGS_FOR_TARGET=-O2    -mcmodel=medlow'
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.0.0 20240829 (experimental) (g155da081706) 

$ ./clang -v
clang version 17.0.6 (https://github.com/ruyisdk/llvm-project.git 94384bb00a9bc73ffd2e2df398e602292bf1349c)
Target: riscv64-unknown-linux-gnu
Thread model: posix
InstalledDir: /data/action/osd/riscv/bin/.
Found candidate GCC installation: /data/action/osd/riscv/bin/./../lib/gcc/riscv64-unknown-linux-gnu/13.2.0
Found candidate GCC installation: /data/action/osd/riscv/bin/./../lib/gcc/riscv64-unknown-linux-gnu/15.0.0
Selected GCC installation: /data/action/osd/riscv/bin/./../lib/gcc/riscv64-unknown-linux-gnu/15.0.0

$ ./riscv64-unknown-linux-gnu-gcc -march=rv64gc_xtheadvector -dM -E - < /dev/null | sort | grep __riscv
#define __riscv 1
#define __riscv_a 2001000
#define __riscv_arch_test 1
#define __riscv_atomic 1
#define __riscv_c 2000000
#define __riscv_cmodel_medlow 1
#define __riscv_compressed 1
#define __riscv_d 2002000
#define __riscv_div 1
#define __riscv_f 2002000
#define __riscv_fdiv 1
#define __riscv_flen 64
#define __riscv_float_abi_double 1
#define __riscv_fsqrt 1
#define __riscv_i 2001000
#define __riscv_m 2000000
#define __riscv_misaligned_slow 1
#define __riscv_mul 1
#define __riscv_muldiv 1
#define __riscv_th_v_intrinsic 11000
#define __riscv_v_elen 64
#define __riscv_v_elen_fp 64
#define __riscv_v_intrinsic 12000
#define __riscv_v_min_vlen 128
#define __riscv_vector 1
#define __riscv_xlen 64
#define __riscv_xtheadvector 1000000
#define __riscv_zaamo 1000000
#define __riscv_zalrsc 1000000
#define __riscv_zca 1000000
#define __riscv_zcd 1000000
#define __riscv_zicsr 2000000
#define __riscv_zifencei 2000000

$ ./clang -march=rv64gc_xtheadvector -dM -E - < /dev/null | sort | grep __riscv
#define __riscv 1
#define __riscv_a 2001000
#define __riscv_arch_test 1
#define __riscv_atomic 1
#define __riscv_c 2000000
#define __riscv_cmodel_medlow 1
#define __riscv_compressed 1
#define __riscv_d 2002000
#define __riscv_div 1
#define __riscv_f 2002000
#define __riscv_fdiv 1
#define __riscv_flen 64
#define __riscv_float_abi_double 1
#define __riscv_fsqrt 1
#define __riscv_i 2001000
#define __riscv_m 2000000
#define __riscv_mul 1
#define __riscv_muldiv 1
#define __riscv_th_v_intrinsic 11000
#define __riscv_v_elen 64
#define __riscv_v_elen_fp 64
#define __riscv_xlen 64
#define __riscv_xtheadvector 1000000
#define __riscv_zicsr 2000000
#define __riscv_zifencei 2000000