Implement UTF16 to UTF8 length with replacement for rvv architecture - #890

> ./build/benchmarks/benchmark  -P utf8_length_from_utf16le  utf8_length_from_utf16le_with_replacement   -F ~/emoji.txt -I 100000
We define the number of bytes to be the number of *input* bytes.
We define a 'char' to be a code point (between 1 and 4 bytes).
Using ICU version 78.1
Using iconv version 267
Compiler: Clang 16.0.0
SIMDUTF version: 7.7.1
System: arm64
===========================
testcases: 1
input detected as UTF8
===========================
utf8_length_from_utf16le+arm64, input size: 3150, iterations: 100000, dataset: ~/emoji.txt
  16.573 GB/s (5.1 %)   16.573 Gc/s     1.00 byte/char     95.0 ns
utf8_length_from_utf16le_with_replacement+arm64, input size: 3150, iterations: 100000, dataset: ~/emoji.txt
  12.472 GB/s (3.0 %)   12.472 Gc/s     1.00 byte/char    126.3 ns

$ qemu-riscv64-static   -L /lib/riscv64-linux-gnu   -E LD_LIBRARY_PATH=/opt/icu-riscv64/lib:/lib/riscv64-linux-gnu   ./build/benchmarks/benchmark -P  utf8_length_from_utf16le  utf8_length_from_utf16le_with_replacement   -F ~/emoji.txt -I 100000
We define the number of bytes to be the number of *input* bytes.
We define a 'char' to be a code point (between 1 and 4 bytes).
Using ICU version 74.2
Compiler: Clang 18.1.3
SIMDUTF version: 7.7.1
System: rvv
===========================
testcases: 1
input detected as UTF8
===========================
utf8_length_from_utf16le+rvv, input size: 3150, iterations: 100000, dataset: ~/emoji.txt
   0.043 GB/s (9.7 %)    0.043 Gc/s     1.00 byte/char  36709.0 ns
utf8_length_from_utf16le_with_replacement+rvv, input size: 3150, iterations: 100000, dataset: ~/emoji.txt
   0.010 GB/s (1.8 %)    0.010 Gc/s     1.00 byte/char 150793.0 ns

$ qemu-riscv64-static   -L /lib/riscv64-linux-gnu   -E LD_LIBRARY_PATH=/opt/icu-riscv64/lib:/lib/riscv64-linux-gnu   ./build/benchmarks/benchmark -P  utf8_length_from_utf16le  utf8_length_from_utf16le_with_replacement   -F ~/emoji.txt -I 100000
We define the number of bytes to be the number of *input* bytes.
We define a 'char' to be a code point (between 1 and 4 bytes).
Using ICU version 74.2
Compiler: Clang 18.1.3
SIMDUTF version: 7.7.1
System: rvv
===========================
testcases: 1
input detected as UTF8
===========================
utf8_length_from_utf16le+rvv, input size: 3150, iterations: 100000, dataset: ~/emoji.txt
   0.042 GB/s (1.6 %)    0.042 Gc/s     1.00 byte/char  37250.0 ns
utf8_length_from_utf16le_with_replacement+rvv, input size: 3150, iterations: 100000, dataset: ~/emoji.txt
   0.450 GB/s (4.6 %)    0.450 Gc/s     1.00 byte/char   3500.0 ns