Fix build with GCC 10 when long double = double.
On platforms where long double has the same ABI as double, glibc defines long double functions as aliases for the corresponding double functions. The declarations of those functions in <math.h> are disabled to avoid problems with aliases having incompatible types, but GCC 10 now gives errors for incompatible types when the long double function is known to GCC as a built-in function, not just when there is an incompatible header declaration. This patch fixes those errors by using appropriate -fno-builtin-<function> options to compile the double functions. The list of CFLAGS-* settings is an appropriately adapted version of that in sysdeps/ieee754/ldbl-opt/Makefile used there for building nldbl-*.c files; in particular, the options are used even if GCC does not currently have a built-in function of a given function, so that adding such a built-in function in future will not break the glibc build. Thus, various of the CFLAGS-* settings are only for future-proofing and may not currently be needed (and it's possible some could be irrelevant for other reasons). Tested with build-many-glibcs.py for arm-linux-gnueabi (compilers and glibcs builds), where it fixes the build that previously failed. (cherry picked from commit 49348bea)
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