[libc] Use get_round() instead of floating point tricks in generic hypot implementation.

[lntue]

Summary

The floating point tricks used to get rounding mode require -frounding-math flag, which behaves differently on aarch64. Reverting back to use get_round instead.

libc/src/__support/FPUtil/Hypot.h

static inline T hypot(T x, T y) {
   uint16_t a_exp, b_exp, out_exp;
   UIntType a_mant, b_mant;
   DUIntType a_mant_sq, b_mant_sq;
   bool sticky_bits;
 
   if ((x_bits.get_unbiased_exponent() >=
        y_bits.get_unbiased_exponent() + MantissaWidth<T>::VALUE + 2) ||
       (y == 0)) {
-    // Check if the rounding mode is FE_UPWARD, will need -frounding-math so
-    // that the compiler does not optimize it away.
-    if ((y != 0) && (0x1p0f + 0x1p-24f != 0x1p0f)) {
+    if ((y != 0) && (get_round() == FE_UPWARD)) {
       UIntType out_bits = FPBits_t(abs(x)).uintval();
       return T(FPBits_t(++out_bits));
     }
     return abs(x);
   } else if ((y_bits.get_unbiased_exponent() >=
               x_bits.get_unbiased_exponent() + MantissaWidth<T>::VALUE + 2) ||
              (x == 0)) {
-    // Check if the rounding mode is FE_UPWARD, will need -frounding-math so
-    // that the compiler does not optimize it away.
-    if ((x != 0) && (0x1p0f + 0x1p-24f != 0x1p0f)) {
+    if ((x != 0) && (get_round() == FE_UPWARD)) {
       UIntType out_bits = FPBits_t(abs(y)).uintval();
       return T(FPBits_t(++out_bits));
     }
     return abs(y);
   }
 
   if (abs(x) >= abs(y)) {
     a_exp = x_bits.get_unbiased_exponent();

libc/src/math/generic/CMakeLists.txt

add_entrypoint_object(
   SRCS
     hypotf.cpp
   HDRS
     ../hypotf.h
   DEPENDS
     libc.src.__support.FPUtil.fputil
   COMPILE_OPTIONS
     -O3
-    -frounding-math
-    -Wno-c++17-extensions
 )
 
 add_entrypoint_object(
   fdim
   SRCS
     fdim.cpp
   HDRS
     ../fdim.h
add_entrypoint_object(
   SRCS
     hypot.cpp
   HDRS
     ../hypot.h
   DEPENDS
     libc.src.__support.FPUtil.fputil
   COMPILE_OPTIONS
     -O3
-    -frounding-math
-    -Wno-c++17-extensions
 )
 
 add_entrypoint_object(
   nextafter
   SRCS
     nextafter.cpp
   HDRS
     ../nextafter.h