options/glibc: Add __riscv_flush_icache() #4239

Job	Run time
Compile sysdeps (dripos)	2m 24s
Compile sysdeps (lemon)	2m 26s
Build mlibc (x86_64, mlibc, gcc)	3m 46s
Build mlibc (x86_64, mlibc-static, gcc)	3m 13s
Compile sysdeps (aero)	2m 21s
Build mlibc (x86_64, mlibc-shared, gcc)	3m 12s
Compile sysdeps (ironclad)	2m 47s
Build mlibc (x86_64, mlibc-ansi-only, gcc)	2m 55s
Compile sysdeps (lyre)	2m 47s
Build mlibc (x86_64, mlibc-headers-only, gcc)	2m 4s
Compile sysdeps (keyronex)	2m 20s
Build mlibc (riscv64, mlibc, gcc)	3m 51s
Compile sysdeps (managarm)	3m 6s
Build mlibc (riscv64, mlibc-static, gcc)	4m 37s
Compile sysdeps (vinix)	2m 18s
Build mlibc (riscv64, mlibc-shared, gcc)	3m 37s
Compile sysdeps (astral)	2m 23s
Build mlibc (riscv64, mlibc-ansi-only, gcc)	2m 55s
Build mlibc (riscv64, mlibc-headers-only, gcc)	2m 6s
Build mlibc (aarch64, mlibc, gcc)	4m 31s
Build mlibc (aarch64, mlibc-static, gcc)	3m 24s
Build mlibc (aarch64, mlibc-shared, gcc)	3m 44s
Build mlibc (aarch64, mlibc-ansi-only, gcc)	3m 11s
Build mlibc (aarch64, mlibc-headers-only, gcc)	2m 3s
Build mlibc (x86, mlibc, gcc)	4m 8s
Build mlibc (x86, mlibc-static, gcc)	3m 19s
Build mlibc (x86, mlibc-shared, gcc)	3m 17s
Build mlibc (x86, mlibc-ansi-only, gcc)	2m 58s
Build mlibc (x86, mlibc-headers-only, gcc)	2m 1s
Build mlibc (m68k, mlibc, gcc)	3m 57s
Build mlibc (m68k, mlibc-static, gcc)	2m 58s
Build mlibc (m68k, mlibc-shared, gcc)	3m 6s
Build mlibc (m68k, mlibc-ansi-only, gcc)	2m 52s
Build mlibc (m68k, mlibc-headers-only, gcc)	2m 1s
Build mlibc (x86_64, mlibc, clang)	3m 25s
	1h 46m 3s

Provide feedback