x86_64 generic mpn_mul_1c symbol issue

[Amir Aupov]

mpn_mul_1 and mpn_mul_1c in mpn/x86_64/mul_1.asm should be entry points of
one function, not distinct functions.
This confuses downstream binary optimization tool llvm-bolt when it
attempts to inline mpn_mul_1c. The fix is trivial but I can't seem to find
a proper venue to submit it:

```
diff --git a/gmp-6.1.2-generic/mpn/x86_64/mul_1.asm
b/gmp-6.1.2-generic/mpn/x86_64/mul_1.asm
--- a/gmp-6.1.2-generic/mpn/x86_64/mul_1.asm
+++ b/gmp-6.1.2-generic/mpn/x86_64/mul_1.asm
@@ -77,7 +77,6 @@
 IFSTD(`        mov     %r8, %r10')
 IFDOS(`        mov     64(%rsp), %r10')        C 40 + 3*8  (3 push insns)
        jmp     L(common)
-EPILOGUE()
 PROLOGUE(mpn_mul_1)
 IFDOS(``push   %rsi            '')
@@ -180,4 +179,5 @@
 IFDOS(``pop    %rdi            '')
 IFDOS(``pop    %rsi            '')
        ret
-EPILOGUE()
+EPILOGUE(mpn_mul_1)
+EPILOGUE(mpn_mul_1c)
```

Thanks,
-Amir