libc: remove riscv64 mem*/str* ifuncs and fallbacks

Only riscv64 chips with vector support are supported, so the fallbacks &
ifuncs here aren't needed.

Bug: 378117947
Test: lunch aosp_cf_riscv64_phone-trunk_staging-userdebug; m
Change-Id: Idf4aed1acc4f0ceea2c17fc6771052563aec94c1
diff --git a/libc/arch-riscv64/string/strlen_v.S b/libc/arch-riscv64/string/strlen_v.S
index c284021..7f7d2dd 100644
--- a/libc/arch-riscv64/string/strlen_v.S
+++ b/libc/arch-riscv64/string/strlen_v.S
@@ -66,7 +66,7 @@
 #define vStr v0
 #define vMaskEnd v2
 
-ENTRY(strlen_v)
+ENTRY(strlen)
 
     mv pCopyStr, pStr
 L(loop):
@@ -84,4 +84,4 @@
 
     ret
 
-END(strlen_v)
+END(strlen)