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/memchr_v.S b/libc/arch-riscv64/string/memchr_v.S
index d4999c3..8833436 100644
--- a/libc/arch-riscv64/string/memchr_v.S
+++ b/libc/arch-riscv64/string/memchr_v.S
@@ -68,7 +68,7 @@
 #define vData v0
 #define vMask v8
 
-ENTRY(memchr_v)
+ENTRY(memchr)
 
 L(loop):
     vsetvli iVL, iNum, e8, ELEM_LMUL_SETTING, ta, ma
@@ -93,4 +93,4 @@
     add iResult, pSrc, iTemp
     ret
 
-END(memchr_v)
+END(memchr)