Update toolchain to support std::remainder (esp8266#7849)

Update newlib to enable the __ieee754_remainder(f) calls required by
std::remainder and others.

Add device test for std::remainder variants.

Fixes esp8266#7845

Author: earlephilhower

bootloaders/eboot/eboot.elf

@@ -1,4 +1,5 @@
 #include <BSTest.h>
+#include <cmath>
 BS_ENV_DECLARE();
 
 
@@ -30,6 +31,13 @@ TEST_CASE("#612 fmod and sqrt work", "[newlib]")
 	CHECK(fabs(fmod(-10, -3) - (-1.0)) < 1e-5);
 }
 
+
+TEST_CASE("#7845 std::remainder works", "[newlib]")
+{
+        CHECK(fabs(std::remainder((double)10.123456, (double)5.0) - (double)0.123456) < 1e-5);
+        CHECK(fabs(std::remainder((float)15.123456, (float)5.0) - (float)0.123456) < 1e-5);
+}
+
 void loop()
 {
 }