[libc] Fix signed zeros for exp10m1f16 and tanhf16. (llvm#116654)

Author: lntue

libc/src/math/generic/exp10m1f16.cpp

@@ -119,6 +119,9 @@ LLVM_LIBC_FUNCTION(float16, exp10m1f16, (float16 x)) {
 
     // When |x| <= 2^(-3).
     if (x_abs <= 0x3000U) {
+      if (LIBC_UNLIKELY(x_abs == 0))
+        return x;
+
       if (auto r = EXP10M1F16_EXCEPTS_LO.lookup(x_u);
           LIBC_UNLIKELY(r.has_value()))
         return r.value();

libc/src/math/generic/tanhf16.cpp

@@ -64,6 +64,9 @@ LLVM_LIBC_FUNCTION(float16, tanhf16, (float16 x)) {
 
     // When |x| <= 0x1.d2p-4.
     if (x_abs <= 0x2f48U) {
+      if (LIBC_UNLIKELY(x_abs == 0))
+        return x;
+
       float xf = x;
       float xf_sq = xf * xf;
       // Degree-7 Taylor expansion generated by Sollya with the following