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/memcpy_v.S b/libc/arch-riscv64/string/memcpy_v.S
index 93ec60f..def1d9b 100644
--- a/libc/arch-riscv64/string/memcpy_v.S
+++ b/libc/arch-riscv64/string/memcpy_v.S
@@ -65,7 +65,7 @@
 #define ELEM_LMUL_SETTING m8
 #define vData v0
 
-ENTRY(memcpy_v)
+ENTRY(memcpy)
 
     mv pDstPtr, pDst
 
@@ -82,4 +82,4 @@
 
     ret
 
-END(memcpy_v)
+END(memcpy)