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/strchr_v.S b/libc/arch-riscv64/string/strchr_v.S
index bc7b58a..ea13c5d 100644
--- a/libc/arch-riscv64/string/strchr_v.S
+++ b/libc/arch-riscv64/string/strchr_v.S
@@ -69,7 +69,7 @@
 #define vMaskEnd v8
 #define vMaskCh v9
 
-ENTRY(strchr_v)
+ENTRY(strchr)
 
 L(strchr_loop):
     vsetvli iVL, zero, e8, ELEM_LMUL_SETTING, ta, ma
@@ -91,4 +91,4 @@
     add pStr, pStr, iChOffset
     ret
 
-END(strchr_v)
+END(strchr)