Use std::abs for a float-compatible abs() function (#2738)

* Other Arduino cores uses a macro to redefine libc abs() to take any
  type, meaning abs(-3.3) == 3.3 not the normal libc result of 3.

* 1e4bf14a3 (#1783) replaced similar min, max macros with c++ stdlib. However
  this change includes <algorithm> after the line which defines the abs() macro.
  <algorithm> includes <cstdlib> which undefines abs() and re-defines it.

* This means abs() becomes the plain libc version again which only takes
  integers, so abs(-3.3) == 3. As reported here:
  https://github.com/espressif/esp-idf/issues/3405

This fix tries to keep in the spirit of #1783 by using libstdc++. The other
option would be to include <cstdlib> before defining the abs() macro, so it
doesn't get undef-ed again later on.

cores/esp32/Arduino.h

@@ -68,12 +68,6 @@
 #define __STRINGIFY(a) #a
 #endif
 
-// undefine stdlib's abs if encountered
-#ifdef abs
-#undef abs
-#endif
-
-#define abs(x) ((x)>0?(x):-(x))
 #define constrain(amt,low,high) ((amt)<(low)?(low):((amt)>(high)?(high):(amt)))
 #define radians(deg) ((deg)*DEG_TO_RAD)
 #define degrees(rad) ((rad)*RAD_TO_DEG)
@@ -160,6 +154,7 @@ void shiftOut(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder, uint8_t val);
 #include "HardwareSerial.h"
 #include "Esp.h"
 
+using std::abs;
 using std::isinf;
 using std::isnan;
 using std::max;